You're describing the problem, it's setting, and your platform very vaguely. But from general principles, it's best to charge in the least noticeable way possible, so a % of the seller's commission would be the best method.

That way it's invisible until a sale comes through, and even then, it occurs automatically as a deduction. If you charge a subscription fee you're requiring the seller to actively pull out their card and send you money, a barrier that's much harder to get them to cross than the less noticeable, passive and automatic deduction from a sale.

If you'd like to discuss the issue with respect to the specifics of your particular platform and its market, let me know and we can set up a call,

best,

Lee