What is the "ceiling" for pricing used vinyl/records?

I was at a local store (see RSD Best Selling thread ) this week , and the store owner was explaining , it is getting very difficult to determine a "ceiling" on how high he can go on certain used albums/artists ...it seems with certain artists (or albums) no matter how high he goes , they still within a week or so....the used market is so odd , right now ???

 

The ceiling changes constantly. I'm amazed at what some records go for. But I remember the days when doowop 45s were gold--many can be had for a relative song now (no pun intended), so what goes up, etc.