You can buy and sell Namecoins here:

Decentralized exchanges
Bisq

Gold level exchanges

Silver level exchanges

Bronze level exchanges

Basic level exchanges
AltCoinTrader
Altilly
Coingi
Hodl Hodl
Livecoin
NLexch
QBTC
SouthXchange

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