Yes quilter1943...my retail store had this problem a few years ago. We donated a bunch of lotions and lo and behold, a lady tried to return about 10 bottles for different stuff! We had marked the bottoms, and she had even tried to erase some of the marks...and then she got huffy when we wouldn't do the exchange!
Another thing with selling at 90 % off...2 part...if customers learn to depend on this markdown...they only will buy the markdown stuff...and not buy anything else (YES this happens). If customers learn we donate stuff to a certain restore....they don't come to our location....they go to the restore.
I used to get so mad when the local Joanns wouldn't donate to the local school PTOs, until the manager reminded me that there were 21 grade schools in the area.....