I regularly buy gift cards on the second-hand market for almost all of the stores I shop at regularly. The major marketplaces have strong buyer protections and will take care of you if there is any issue with the card, provided you inform them within the guarantee period (usually 45 or 60 days). After hundreds of cards purchased, I've had only 2 duds, both of which were refunded without question.

Costco cards are hard to obtain on these sites because they get snatched up quickly by people who want to shop without a membership. They are easy to purchase on eBay, but usually for above face value and in small increments, so that wouldn't be useful.

Costco.com now accepts every card but Amex, so I don't know a way around this. I'd be inclined to shop at another store to meet the MSR.