For starters list them accepting offers at
www.afternic.com
www.sedo.com
www.tdnam.com
You will usually want to set a minimum offer price at $100 or so. Setting it low will encourage more offers. Most importantly don't think every name is worth a million but don't sell to cheap. Reinvest as much as you can. Don't drain your customers pockets and they will come back for more and more. And last but not least don't get discouraged if your names don't sell. It's a numbers game just as it is in any business. The more names you have and the more places you promote them for sale the more offers you will receive. The more names you sell the more names you can buy. Buy low, sell high, and continue..