If it hasn't been long just go in and talk to them. The receipt won't matter either way except maybe to remind them of the sale if some time has passed. Used parts usually aren't warrantied, but if it's a fairly recent job, a decent shop would try to correct the problem with another used crank, or give you some sort of credit towards a new one.

In my early years in the bike biz, I was the go to guy for problems, and I assure you that customers who came in with an honest explanation of their problem and asked if I could help them, did far better than those who came in with a negative attitude. I saw problems as an opportunity to showcase out pro customer attitude, but if I felt that no matter what I did, we'd be reviled anyway, I wasn't inclined to do much.