Marketplace pricing, percentage based fee or monthly payment?

I'm working on a simple marketplace, like a Gumroad style digital download store. I'm wondering if anyone has any research into what works better, a flat rate monthly subscription i.e. $14.99/month (+2% payment provider processing fee) or a percentage based transaction fee i.e. 5% per purchase (+2% payment provider processing fee).

The pro's to the fees are that users can get started for free, and they wont have to worry about spending any money unless they're making money, which I think would greatly reduce the barrier to entry. However, if the seller is very successful then it could quickly add up to a lot of fees, which is a big pro for me, but a big con for the seller.

The pros to the subscription is obviously that the seller knows exactly what they're going to be paying, and I will get paid no matter what. However, the cons like I said before is that there will be a barrier to entry where the sellers might now join if they're not 100% sure they can sell at least the monthly fee worth.

I can of course implement both options like Gumroad does but at least for the start I'd like to concentrate on one option.

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