Why does any animal try a behavior? At some point, it worked. The offered an item outside the stated criteria and someone bought it.
I think what people are sometimes doing is trying to discover how rigid (or point out how unrealistic, as others say) the original person is in their requirements.
I posted an ISO ad with height range, age range, ability (eventing), geographical area and price range. I specified no OTTBs. I got relentless ads for short OTTBs with no jumping exp, on the other side of the country. “not even close AND no cigar!” became my cry. I gave up asking that group of people. Tho one particularly helpful friend was unrelenting about the OTTB part until I told her two or three times in great detail WHY I would not consider one, and I think it’s sunk in 
Over time, with no luck finding a horse for sale but having tried many, my height and age restriction has become slightly narrower (no less than 16h, sticked, but will look at 3 yr olds now) and my price range higher. My tolerance for nasty attitudes is zero, but my geographic location range has expanded a lot.
So, if someone had come at me with a 3yr mare in WA with 10 days under saddle 4 months ago I would have said no thanks. Now I’m begging for video from the seller and trying to decide if I should risk my life by flying or drive 12 hours each way…