You can buy and sell Namecoins here:

Decentralized exchanges
(uses Tor by default; memory-safe)
(uses atomic trades)
(uses atomic trades; memory-safe)

Gold level exchanges

Silver level exchanges

Bronze level exchanges

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

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