'Tis a simple suggestion really. Allow items bought from mementos to also be sold back to the vendor for a portion of the memento's spent.
For example, when an item is bought from a vendor, the item can be sold back for a tenth of the original purchase price in gold. My suggestion is this: Allow items that are bought with mems, to also be sold back for a tenth of the memento purchase price.
For example: You buy a Belathis Speed Necklace from the Sarlo memento vendors for 980 memento's and 25,830 gold. It comes out 72 dura. You can't really do anything with that so you sell it back to the vendor. You receive 2,583 gold and 98 memento's. It's not a large amount by any means but it's just a small return that you won't have to farm again as you keep continuously going for OD.
Now of course this would only work for items that cost more than 10 memento's, but as that's nearly everything I don't see that as a problem.