depending on the tools you own and your skill level,be careful of buying a used bike that needs a lot of wear items replaced. Be on the lookout for something that was ridden on the trainer a lot and has sweat down in the headset, check carefully for play in the bottom bracket and wheel bearings, avoid bikes with filthy chains, cables without little end caps, plier marks on bolts, and other indicators of abuse. If you have to drag a bike into the shop to have these items repaired it gets expensive in a hurry, like many hundreds.

On aluminum bikes with internal headsets, be careful of damage to the internal cups that leaves the headset with some play, in that case the frame is shot. Also look carefully for cracks in the welds and parts secured to the frame with rivets coming loose like the FD mount and fractures around the bottle cage mounts.

One indicator of a bike ridden by a real sweat bag is the condition of the bar tape. Some people ride on trainers and sweat so much the bars corrode, avoid bikes like that