Accs-market (remember the hyphen, the owner of the one without the hyphen was banned from here for scamming lmao) seems to be the most popular and have the widest variety. Just make sure to use escrow service and ignore dms asking you to go off platform. When I posted my channel there I got like 3 "buyers" asking me to message them on Telegram. They have channels of any subs like 500-500k
Fameswap seems to have people selling them for much more money than other sites, and seems to be more for big channels. So if your channel is big, you should try it since people might be willing to pay more for it there, but it doesn't have such high SEO ranking.
Also DO NOT accept payments on PlayerUp, whatever you do. They often hold your money hostage sometimes forcing an ID verification or other BS even just for the buyer. People often buy directly from the seller on there on Telegram Discord etc for that reason, which is of course riskier for the buyer. They made people take their down reviews even bribing them on Trustpilot before their profile got terminated. They paid me $25 in credits to take down my review a couple years ago XD
