The most important thing to look for when buying reconditioned power tools is a warranty. Not only should there always be a warranty, but the terms of the warranty should be appropriately comparable to the warranty of the same tool when bought new.
Another important factor is the reason that the tool was reconditioned, and how it was reconditioned. The most ideal scenario is to buy a tool that was minimally used, or not used at all, as opposed to buying one that was misused or has some flaw. Finally, the price of the reconditioned tool should be notably lower than the price of the tool when bought new, unless it is being sold in good-as-new condition.