well first. make sure its a reputable lfs. i have seen ones that have display and stock tanks covered with red planaria,and when asked about it, they said they didnt consider it a problem! know what you are gettign, how long its been there, the conditions its kept under and if you do decide to buy it, make sure you acclimate it to your tank properly. use the drip method, and never introduce the bag water into the tank!
HTH
good luck
jon