If I were buying a BD bike I'd be doing so knowing that there are potential problem areas. I'd assume that the shifting would need tweaking, I'd check all the cables, I'd check the bottom bracket and headset to make sure it spins smoothly. I'd probably do a full check of every bolt. If I saw any problems with the rotating parts I might take it to a shop, but more likely I'd disassemble it myself and redo it.

The average inexperienced person would not do any of this and have problems.