I took another look at Purse.io just a few hours ago and it looks like if you're a total newbie, you can't choose a discount percentage, and have to buy your order directly through Purse for a few percent off. I'm guessing as your account ages and you have more transactions under your belt, then they'd start to loosen rules and allow you to post your listing and listing price, to choose a percentage off.
I haven't used Purse.io in a while, and they have changed a little, but AFAIK you have to order a certain amount at a fixed discount before you can choose your own discount. I have bought many hundreds of dollars worth of items at 15%-25% discounts through Purse.io and I am happy with their service. I have had no major problems.