You can buy and sell Namecoins here:

Decentralized exchanges
Bisq

Gold level exchanges

Silver level exchanges
bit.AC

Bronze level exchanges

Basic level exchanges
247exchange
24change
alcurEX
Bter
bx.in.th
ExchangeMyCoins
Livecoin
Poloniex
ShapeShift
Tux Exchange
Vircurexuse with caution, see discussion

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