I'm glad to see you here in the Community, @emwats.
You can set up each platform as a Bank account then enter sales and expenses (fee) generated by each source/ store. So you can create a Bank transfer once you transfer funds and can be matched.
Additionally, fees and other charges generated will be recorded. If you have an invoice and you'll need to receive a payment you can choose and use those bank account (Mercari, Poshmark, and eBay).
Thus, I still suggest seeking help from your accountant. He/she could escort you on which option to take based on your recording practice and business needs.
Also, in QuickBooks Online (QBO) you have the option to include a service fee for products or services you sell to customers. Please refer to this article link for future reference: Add service fees manually to invoices.
I'll be keeping an eye out for your response on this. Have a good one.
