There is no reliable single percentage. A shop may pay relatively close to recent sales for a liquid card it can resell quickly, much less for slow or volatile inventory, and very little for bulk that requires hours of sorting. Condition, authenticity, local demand, current stock, platform fees, taxes, and the risk of returns all affect the offer. Active asking prices are not the same as completed sales, and the highest online listing is not a realistic retail benchmark.
Find category-appropriate buyers through the local card shop directory and ask each one to explain the important parts of the offer. For a mixed collection, request separate numbers for key cards, sealed product, and the remaining bulk; that makes comparisons more useful than one unexplained total. Trade credit may be higher because the store keeps the transaction in inventory, but it only has full value if you already want fairly priced stock there. Read the guide to selling a collection before an appointment. A transparent buyer should let you decline, retrieve your cards, and take time to compare without changing the terms as punishment.