The forum place used to be Dnforum but today, I'd recommend:
NamePros - Buy, Sell, Discuss Domain Names
Forget Afternic or Sedo, both have just gone downhill many years ago. Godaddy and Namecheap auctions are not bad - same for Namejet and Flippa. Note that if you sell through a forum, you are invariably reaching traders and not end users. End users will come to you directly through your whois data if they like your Domain Name.