Just sign up for an exchange and have the Namecoins sent to a deposit address generated from your exchange account. Convert them to Bitcoins on the exchange.
I have been doing this with Cryptsy for some time now and have not had a problem.
I second this suggestion. Cryptsy is an excellent choice for this although any altcoin exchange that has NMC listed as one of their currencies (most do) such as Poloniex or Bittrex can work just as well.
However, if you don't want to go through the hassle of registering an account and linking it to your email address, you can also use ShapeShift.io instead. Simply select the "NMC to BTC conversion" option and paste your Bitcoin address in the right-hand box. The site will automatically generate a valid NMC address which you can send namecoins to. Once the site receives namecoins, it will convert these to bitcoins and deposit them directly into your BTC address:
http://www.shapeshift.io/