1) The people who deal in these are usually shady characters who deal in all sorts of stolen goods. You have to know if there was a shinui r’shus.
2) No, but it makes them koihns.
3) Are they karkaos? Are they avodim? Are they shtaros? If the answer to any of the above is “yes”, then you should trade them in for something not in any of those categories.