Lesser Geas (or Geas) the borrower, at their own cost, to return the item in 7 days? Boy, that would sure cut back on overdues at the library.
I have some questions about the basic concept, though. Why would anyone keep doing this? If they have a stock of valuable items, why NOT sell them all and be done with it, if there is a market? What is their motive for having such expensive stock, and taking in a small income instead of selling and retiring? How did they afford such stock in the first place, and why did they sink all that money (and somebody’s XP) into a business, instead of simply, again, living off of such fantastic wealth?
It seems like something that adventurous types might wish existed, but I can’t see a strong motive on the other side for someone to provide it. I tend to feel that way a bit about the whole magic item store idea, mind you, so I am not saying this is way worse than that, only that maybe it requires a little extra background explanation. Like perhaps a very well-meaning group of wizards provides this service if they approve of the intended use of the items. Maybe it is even more interesting if it is a not well-meaning group that rents out magic items.