Are the prices competitive with the used market? Just one datapoint: I know someone who tried to trade in a laptop; Apple offered ~$200, eBayers were buying for ~$400.

No there’s a big gap, but it is meant for people who want a guaranteed amount without going through the trouble of selling it privately.

eBay takes a 13%+ cut, then there's shipping, so you're looking at $300 after cost of business. If you're selling a lot, sure, capture that $100 difference. If you're selling one and dealing with buyers, you'll spend at least one hour taking pictures, writing the ad copy, doing client management, going out to ship + gas to get to the post office, risk of buyer return/fraud. Say you're worth $50/hour and you're being very efficient and do all that in one hour, you may end up with a net economic gain of $40-50 after all that. Is it worth it?