Grab your domains at whatever registrar you use. I prefer namecheap and moniker.
There are tons of places you can sell them. Sitepoint, namepros, sedo, forums with a B/S/T section, etc. There are a few other good ones I'm sure I'm forgetting. Just Google "sell domains".
You said you wanted to register them at the cheapest price. If registering a .com for 50 cents less is going to make a huge difference in your ROI...yeah, you see where I'm going with this.
I'm no domainer, but keep in mind that you'll generally get a much higher price for a domain if you develop even a minimally functional site on it.
Lord Brar has a great blog about domaining:
How I “Really” Make Money From Domain Names | Domaining Tips