ARCs turn up constantly at library book sales because librarians receive them in volume. They usually have plain or simplified covers, a printed line reading 'Uncorrected Proof' or 'Advance Reader's Copy — Not for Sale', and sometimes a publication date and marketing plan printed on the back.
Because they carry a not-for-sale notice, major marketplaces including Amazon prohibit listing them, and eBay restricts them. Selling ARCs can put a seller account at risk, so they are best treated as reading copies or giveaways.
There is a narrow collector market for ARCs of books that later became notable, particularly signed proofs, but that value is realized through specialist channels rather than general marketplaces.