This isn't stated anywhere in the GTS Guide, so I have to ask: If I
trade a pokemon to someone, do they have to trade a pokemon to me
or can they just offer money/items/etc? Likewise, do the number of
pokemon have to be the same on both sides?
They do have to match the same number of Pokemon, likely because of
scams happening, but you can also put items in. Hopefully this
answers your question!