But porphyre77 left a comment that gave me an idea:
I could maybe give some XP for magical objects that are identifies "the hard way" i-e tested by the characters.Suppose you did this: every unsold item gives a flat XP reward of 100 XP per dungeon level, as long as it is used or displayed in a character's lair. If sold, deduct this value from the price received for net experience gained. If used before the character returns from the adventure (trial and error identification,) double the reward if the item is magical and not cursed. Thus, by taking a risk, players might get more XP -- or nothing at all.