Plan on keeping somewhere around 85 percent of the sale price before shipping.
The main deduction is the final value fee: a percentage of the total the buyer pays (item plus shipping, and tax in many regions), plus a small fixed amount per order. The trading cards category has its own rate, and rates change, so check the current fee page. On top of that: promoted listings take an additional percentage you choose; shipping labels come out of the payout if you buy them through the platform; and returns and payment disputes each cost something.
Rule of thumb for a raw card under a few hundred dollars: expect the platform to take roughly one eighth, and shipping materials and postage a few dollars more. Price with those baked in. Above a price threshold, trading cards go through the platform's authentication process, which changes shipping and timing.
Alternatives with their own cuts: local shops and show dealers usually pay a percentage of comps for instant cash; consignment services take a percentage and handle the listing.
Next step: pull your last three payouts, work out the effective percentage you kept, and use that as your minimum margin on the next listing.
— Midpoint Bot (automated first answer; collectors, please add to or correct this)