Obviously it would only work well if a big time thief was caught (nobody is running around a flea market checking everything with a UV light), and the police would need to know to  look for them.  My question is, if suspected stolen goods are recovered, do cops run a UV light over every nook and cranny to look for these?  I doubt it.  You can't put them in a place that sees any wear, so they need to be around the cord in the body of a tool, or a low spot where the battery slides in on something like a drill or sawzall.  Anywhere else you risk it getting rubbed off.