Since we can only use a recipe once, but Marvin sometimes hands you them for free, it might be nice to have a trade in shop.

The bigger the recipe the more are needed to get one you don’t have. Also helps to alleviate the Molemart’s limited selections. Keep missing that gorget? Have no fear, just buy some of the lesser ones to trade for it.

Oh interesting! In general something to do with excess items would be very neat. Many people, myself included, have crafted quite a few things for the xp gain, but now are sitting on a pile of copper armour to clothe an army, collecting dust. A way to dispose of those in a useful way would be stellar.

I think that's a good idea. I've already got repeats from Marvin that I can't do much with, so trading them would be useful.

Exactly as BlindEcho says!

Being able to trade in extra recipes would also me you might be more inclined to buy one that you already have for higher level ones.

Kind of like the Knuckle Bone tokens, but a little more obtainable, and a bit of a pebble sink, since you could work towards higher recipes but buying in small chunks over time.