Found this on a website. Take it for what it's worth:
"The plastic tab that locked the weights in place did not always hold, creating a dangerous situation if it failed while in use. Consumers need to beware of deeply discounted, second hand 2008 model Bowflex SelectTech 552's that might be sold online. The price may look attractive, but the risk of an accident is simply not worth the savings. The problem was corrected by the manufacturer in 2009, which is the model that is currently offered."
Article Source: http://EzineArticles.com/6802680