I am with GG on this one.
At the end of the day Pat you are currently in the possession of stolen property.
No matter how they came about being in whoever's possession. You KNOW that they were stolen originally from YOUR client, and as such remain stolen, no matter how many hands they pass through.
The fact that you have now posted this in a public forum, could actually used against you - particularly from a competitor, or at worst, some legal eagle that wants a win.
Yes business is business, yes we all have to make a living... but that is difficult to do when you are either in jail, or have a rap sheet and known as a person who turns a blind eye to criminal activity.
Pat I know you have a concience. Do the right thing, protect your integrity, your name, your business and your family and do only what is right.
In terms of "what to do" - that's hard. You could call the cops. You could call the original owner - that would be doing the right thing - they are his by law, but that would obviously upset the person that wants them re-lettered and you may find your shop damaged or worse - burnt down.
Personally - I would call the person who dropped them in, ask them to drop by for a "chat" and tell them that you are not accusing them, but are aware that they are stolen, and prefer to pass on the job. I would also call your "client" and just let them know that for confidentiality and possible repercussion reasons you can't divulge their identity but you have had his / her
signs in your shop, recognized them and asked for them to be removed immediately from your premises.
I don't know how much you would make from this - let's say $500. Is it all worth it?
How much would a lawyer cost to engage to defend yourself?
Look at the big picture my friend.
Cheers - G