Travis "T-Bone" Turner offers tips on buying a used bow.
The most critical step in buying a used bow is to first make sure the bow you're considering fits you. Draw length and draw weight should match your needs -- if not, you may end up spending as much on cams and modules as you did on the bow itself.
Inspect the strings and cables. Replacing a string isn't difficult but it will add to the cost of the bow and can also indicate misuse.