The issue for me is that replacing a locker item is pure grinding - enter Venture Cave with 25 gold and scout shops, buy the item if it's available, else repeat. That's a very tedious process, and serves no purpose other than to be tedious. The only exceptions are items that aren't available in shops, of which only four are worth lockering (Avatar Mark, Namtar's Ward, Dragonshield, Sensation Stone), and if you're at the point where you've beaten those dungeons and are willing to discard their treasure from your locker, beating them again is less of a challenge and more of a grind. "Either accept what you have on hand, or grind more" is not, to me, a meaningful decision - or at least, it's not the sort of meaningful decision that I associate with Desktop Dungeons.
I like Darvin's idea. I'd like it even more if you could simply go shopping for locker items directly from the kingdom, without having to enter a dungeon. As I've noted previously, it's especially frustrating during beta testing, when I'll often want to test a recently changed item or see if a particular combination is overpowered, except that the item in question isn't in my locker.