Respective Buyer,
Though you only want to hire those freelancer who have done work like yours, and to be honest I am a newbie, but I want to take one chance.
As you said that you only pay if someone is able to sell at least 5 dresses, so here is no risk. If I am able to fulfill your requirments, then only pay me.
I will be greatefull to you if you give me a chance.
Thanks