Is the used BC really the BC you want or are you thinking of cost? If you like the BC, it fits properly, and is in good shape then why not, if you really want that back inflate BC on the new rack but are thinking its too expensive then I might suggest you really won't be happy with your "bargain".
Also think of reoccurring maintanace costs. For example some mfg. offer free replacement parts (you still pay for laobr) for annual service as long as you are the original purchaser. On used gear of the same brand you pay for the parts and labor.
Try on different brands and styles, see if you can rent the one your considering, to see how it feels on the water
What Prodiver_ca says!! And since he is an instructor, he is right!
I got the scubapro S-tek because I didn't like the other tech BC like the dive-rite, halcon, or OMS. I was trying to get Aqualung's Apek WTX but that was too expensive at the time. And this is what resulted in me getting the S-tek. But soon I'm going to part with it because I finally got the money to buy the Apek WTX and mom finally got that ordered, but I'm still waiting for the harness and she ordered that on my birthday and that was in November.
Otherwise, if you're buying here like in the classified section, then most of the BC are well taken care. Of course, that may be true or not depending on who is selling it. Anyways, just my two cent.