A scandal in the world of baseball card collecting threatens to undermine the value of a long-established card grading system, has put into question the legitimacy of a prominent marketplace and has raised concerns that some collectors overpaid for expensive cards.

Millions of dollars’ worth of cards are at stake as collectors question purchases and wonder whom they can trust.

The controversy centers on the authenticity of sports cards. To verify their condition, cards are sent to a grading company, which scores them on a scale of 1 to 10. Grading sets the value of the cards and is considered a prerequisite for selling them on an auction platform like eBay.

Cards in pristine condition are highly valued by collectors and can fetch thousands of dollars more than similar cards with scuffs or worn edges. Sellers can improve the appearance of a card by trimming its edges or removing residue, but collectors believe any alterations make a card less authentic, and cards that are known to have been doctored are often worth considerably less.