Me, being a suspicious person, would drill them the following questions:
1): Did someone die on it? - gotta know how deep to clean it.
2): Did someone do 'the nasty' on it? - same excuse, but heavier cleaning.
3): Was this involved in a crime(excluding 1 and 2) - like if someone had a 'stash' in it? Not going to jail because someone offloaded their sins on me, lol!
and:
4): (optional) Did the dog chew/pee on it? But scratches and smell can be buffed out, so not a biggie.
If "no" is answered to at least 3 of the above, I'd at least say "Thank you", depending on what it is.