As the title suggests, it's important to be vigilant when purchasing Micro SD cards - or any flash memory, for that matter, independent of the source - as there exists a large and sophisticated black market in counterfeit memory; in short - as a consumer, you need to trust, but verify.
For example: though your card may state that it's a Samsung 32GB Class 10 Micro SDHC, and your OS (operating system) may even read it as being a 32GB card, and you can (seemingly) load GBs upon GBs into it, and the font and the color and 'Made in Korea' all appear exactly as other cards you've had, the card may be neither the stated size nor the stated class. And, this is a problem.
Think of the fraudsters as pedophile-magicians who traffic in illusions - and also want to steal your money and innocence; and think of yourself, for the moment, as a kid with a cute smile and big piggy bank - because that's the way the pedophile-magician-fraudsters view you. But, you're going to grow up fast - and see beyond the tricks.
We won't 'go gentle into that good night.'
While you may not necessarily be a 'power user' or know or care much about Class and whether the card's writing at (a minimum of) 10MBs/sec, you should be concerned that if the class was mislabeled on the card, then the memory size likely was misstated as well; misstated memory size is significant in that, as you snap away under the impression that you've got 32GBs at your disposal, and you take 1000s of images and GBs and GBs of video of 'Little Johnnie's First Steps' and 'Our Dream Cruise in Greece', those files - let's think of them as Memories - may be corrupted or erased, because your card is really only 8GBs - and all of those images and videos will be gone. Forever. Like the subway 'magician' who just made your wallet disappear.
Remember: the pedophile-magician-fraudsters are relatively sophisticated; the physical characteristics of the drive itself will look similar to others you've purchased; your computer, camera and phone will (be tricked into) read(ing)/display(ing) whatever digital characteristics - drive size, for example - the pedophile-magician-fraudsters want you to believe it is.
That having been said, here's what you need to do: once you've received your card, you'll need to verify 'class' and verify 'size' prior to leaving feedback for the seller. This bears repeating: verify Class and Size prior to leaving feedback for the seller.
How to verify class: For Android phones, there exists a free app by Veluscek Ales - SD Tools - which allows you to see various details on the card, such as Manufacturer, Serial #, OEM ID etc. in addition to running Read and Write tests; pop the SD card into your Droid and fire up the app - if the card is a well-made counterfeit, then you likely won't be able to determine immediately whether it's fake from these listed details; if it's poorly done (for example, some fields are not filled out, the Serial # field is blank etc.), it may be readily obvious. With the details provided however, you can cross-reference the Serial etc., with the purported Manufacturer - the manufacturer will tell you in short time if it is authentic or not.
The litmus test is the Speed Test: run it; if the card states that you've got a Class 10 but consistent tests are Writing only at 6.9 to 7.2, there's likely an issue. Bear in mind: Class is based on minimum standards; cards should perform above these minimums.
There exist other applications for other devices to check speed - or to run from your laptop; check out how online.
How to verify size: the most comprehensive manner is to download the free H2testw (most current version as of writing is V1.4); it's an application written by a clever German guy - and is something of the Gold Standard for testing; it's super easy to use - and gives you the option of running it in either German or English; it will likely run about 4 hours (in function of your card speed) to Test - Write and Verify - but is well worth the investment in time.
In brief, H2testw V1.4 writes packets to the entirety of the drive, until it's full; then, it attempts to read back what it wrote to the drive, and in so doing determines 1) actual drive size and 2) whether there are errors on the drive ie whether it was able to read back without error everything it wrote.
If you've paid for 32GBs and are expecting 32GBs - verify that you've got 32GBs (note: a properly formatted card will actually net only anywhere from 28-29GBs - you will not get 32GBs. If test results shows 12GBs on a 32GB card, you've got a problem.) Everything is not always as it seems.
Bottomline: beware of what you're buying; trust the vendor - but verify the product. And, be cognizant that not all who left positive reviews (either for the product itself or the vendor in particular) thoroughly tested the cards they received - and that failing to do so could subject you to greatly increased chance of loss of important files/Memories - pics, videos, documents - however it is that you use the memory.
Don't let the bad man steal your piggy bank - or defile you, or your files.
:: The below was Added / Edited 10JUN2012 ::
Note that the 3-star rating is based is based on the following:
1) that the card I rec'd and tested was not as labeled ie it was not up to spec, and was of questionable authenticity; this earned it a 1-star rating.
2) that the Vendor who sold it a) was responsive and reasonable, b) appeared to be 'acting in good faith', which is to say that the Vendor did not appear to be knowingly selling non-spec / counterfeit items, as evidenced by our exchange, their overwhelming positive customer feedback and the Vendor's offer to have me test a random sampling of cards, to determine whether mine was an anomaly, consistent with the batch or particular to the manufacturer and model etc. and c) accepted without question the return. This earned the Vendor a 5-star rating.
These two ratings average to 3 stars.
Note the following, as well: retail-packaging on a product does not make it more authentic; the packaging is among the easiest aspects to counterfeit. Furthermore, purchasing a given item fulfilled through / from Amazon benefits the Consumer only with respect to shipping costs and returns: it does not - and should not - imply that a given product is more authentic than another; to assume that Amazon has technical staff testing all products from all Suppliers for authenticity would be foolhardy. Such Amazon fulfillment is a misleading implied endorsement - like an actor in a lab coat on late night television trying to sell pharmaceuticals: in general, we're more wont to lend our trust to an MD in clinical setting than an actor named 'George' from central casting, but the truth is that both may be idiots and we'll find out only after we challenge them a bit.
Additionally: the scale and scope of the counterfeit memory industry is massive; to assume that a given Vendor with a large product line is necessarily complicit in the fraud, either by having 'boots on the ground' in Asia (or elsewhere) or knowingly dealing in counterfeit memory in order to increase margin etc. etc., though fun to think about, is also a bit naive.
The above-noted tools are there to enable you, Gentle Reader / Potential-Consumer, to make an informed decision - and to help you to minimize risk; some will understand this; others won't - and that's okay. To add a bit of Confucius to your day: 'If you know, all has its place; if you don't know, all has its place.'
I have no vested interest in making people understand SDHC cards - or Confucian philosophhy - one way or another.
Do with this insight as you will. But remember, trust, but verify; you may surprise/enlighten yourself and the vendor.