Simple. You don't always get top-dollar through CL or CC. It's not a guarantee.
I've done well with high value books on the big auction sites, but have had some disappointing experiences as well. And by disappointing, I don't just mean low hammer prices, but buyers who have backed out, and a clerical error that kept me from getting paid for 6 months. Once they figured it out they wired me the money that day, but it was concerning. Plus sometimes there's just more hysteria driving up prices on eBay.
Don't get me wrong, I'm not overtly defending eBay. And my wife has instructions that if I get hit by a bus tomorrow to call Vin at CC and sell my collection.
I'm just agnostic when it comes to buying and selling books. There isn't one true place that's universally better. The market is changing too quickly.
Last week I sold a book (2x GPA) thru an Instagram dealer's live auction. 5% consignment fee and was paid in Bitcoin.