Is there an option somewhere to sell multiple copies of your game? I looked but couldn't find any.

I see games on Steam that let you buy one copy for full price, or 4 copies for a discount. (ie $5 for one copy, or $15 for four copies)

It's not something we support right now, but you can buy the same game multiple times to give out as a gift. 

You might be able to use an ownership condition on a sale to give a discount to subsequent purchases: https://itch.io/docs/creators/sales#sale-conditions

Tell me if it doesn't work for you

I was thinking of selling 4 or 10 copies at a discount, to either card traders or gift group members.

They use the extra copies to level up in their gifting group. Or to generate cards. Or I guess just re-sell the keys elsewhere.

The ownership conditions might work, but wouldn't the transaction fees (from paypal or whoever) be much higher because they would be many transactions instead of just one?