A single card or a small lot rarely weighs more than a few ounces, and at that weight the carrier comparison mostly comes down to one number: price per package.
Where USPS wins
Almost everywhere a card seller ships. USPS Ground Advantage is consistently the cheaper option for lightweight packages, includes tracking and up to $100 of insurance by default, and delivers Monday through Saturday at no extra fee. For an eBay order that's a graded card, a small lot, or anything under a pound, Ground Advantage is the default that's hardest to beat on price.
Where UPS wins
Heavier or higher-value shipments. Once a package moves past a few pounds, has to move fast, or needs signature confirmation and higher declared-value coverage as a matter of course, UPS's tracking and claims process is built for that in a way a lightweight-focused USPS service isn't optimized for. It's the better fit for a bulk box shipment, not a single graded card.
What actually decides it
Weight and declared value. Below a pound and under a few hundred dollars in insured value, USPS Ground Advantage wins on price almost every time. Above that, run both rates before choosing; the gap narrows and sometimes flips as weight and coverage go up. A shipping calculator that compares live rates before you commit is worth the two minutes it takes.