Multiple trades available

Could we get a separate quantity for how many times an offer is available?
i.e. I want to sell more than 6 Spreading Sploot - Grace - Long Blade totems, but want to set the price at 1 per (because who is going to buy more than one?) But I don't want to relist it multiple times or just not mark as traded, I'd like to mark 1 as traded and then the quantity drops from 20 to 19 or whatever the number is with the trade offer still being for 1 each