You can buy and sell Namecoins here:

Decentralized exchanges
Bisq (uses Tor by default)
Block DX (uses atomic trades)

Gold level exchanges

Silver level exchanges

Bronze level exchanges
FreeBitcoins.com

Basic level exchanges
QBTC
AltCoinTrader
Hodl Hodl
C-PatEx
Coingi
Livecoin
SouthXchange
NLexch
CoinEx
Altilly
Coinvex

Gold, silver and bronze level exchanges donate to the Namecoin project. The higher the donation the higher the level. Note that the exchanges are not verified in any way. The current fees are 1000/300/100NMC per year. To get on the list contact exchanges a|t namecoin.org