you can return purchases within 30 days by law in most states... They do have the right to keep a record and track your returns and if you do it on a credit card they have the ability of tracking every purchase and return there are companies that do this full time and it is in pretty much full swing now... bring it back!
I took a 3 year old car battery to WM and exchanged it for a new one...no receipt, no ID, no anything.
I did this too but the battery was 3.5 yr old so they prorated it. Did let me get a cheaper replacement so I was only out $1!
Found the battery at the town dump BTW.
This question comes up when people get some really cheap oddball oil somewhere else, like M1 15w50, then "trade it in" at WM. Ethical debates ensue.