I think it should be made possible to let us sell items, such as ocarina’s that we have no use/do not use anymore and ‘maybe’ monsters for silver. Maybe make it so we can ‘feed’ extra monsters we’ve caught to the ones in our party (main monsters we use); which, in result the monster will gain some exp (ranging from 100-500exp for example). If it seems too easy maybe you can also add a ‘fee’ when a player wants to feed a monster to another to gain exp/level.
example 1 (feeding monster “a” to monster “b”): feed monster “a” to monster “b”, which “b” will gain exp ranging from 100-500exp
example 2 (feeding monster “a” to monster “b” with a feed): feed monster “a” to monster “b”; which, depending on high level of “a” or/and “b” the fee will differ in price (1000 silver, 5000 silver, 50 gold, 100gold)
In my opinion this shouldn’t “unbalance” the game that much since you’re not getting that much experience points. Hope this is possible/do-able.