There aren't many exchanges at this time where you can directly sell Lumens for USD.
However, quite a few exchanges exist where you can buy Lumens for other cryptocoins.
For now, I believe a two-step process is required:
- Trade your Lumens for a better-known crypto coin, like Bitcoin, on one of these exchanges: https://coinmarketcap.com/currencies/stellar/#markets
- Transfer those Bitcoins to a well-known exchange that will trade Bitcoin for USD. Coinbase is an obvious example.
As it stands right now, none of this is particularly easy. It can be a bit nerve racking, and cost you quite a bit in transfer fees.
Just so you know ahead of time what to expect :-)