Then I would definitely go with upgradable magic items. Give them magic items, and then give them some currency that allows them to upgrade it: instead of gold pieces, give them some magical particle that can be used to improve your magical items. Regardless of what this currency is (souls, crystals, plants, monster parts), PCs can then complete a certain ritual to improve their magic items, for a cost.

Mechanically, it is exactly the same: instead of selling their +1 sword to buy a +2 sword, they pay in [insert currency] to improve it, and instead of it being in town, they actually are in [whatever place you need to be at for the ritual].