When it comes to selling your domains you have a few options. Some of the methods won't cost anything, others will be low cost and then you have some really effective methods that will require more money upfront as well as percentage of sale. Also many of them even though they have a basic listing for free they give you more exposure with other paid options.
Here are some of the options you have:
Godaddy Auctions - Free upfront with % of sale or more exposure for $20
NameJet Auctions - Free upfront with % of sale
Sedo - Free upfront with % of sale
Flippa - Upfront paid options ranging from $20 to $350 for more exposure plus % of sale
Forum Auctions - Free upfront with % of sale
Park Your Domain and Put A For Sale Banner - Depends on the company but most free
Of course there are other methods but these are the most known and most effective. If this is your first time attempting to sell a domain make sure to dedicate enough time to learning about the above method of selling. There is a ton of information on here that you can find and learn so much from.