I own a few domains I've had for a while.. never really thought about selling them until recently. Any good tips are welcomed. I don't have a specific niche.. just trying to think ahead.
If you have domains already, then definitely do as @moe suggested and list them in the appraisals section .. the feedback there should give you a pulse on the quality of your domains.
Aside from that simply absorb as much information as you can .. while it might seem simple, there's a ton to learn in the world of domains .. mountains of information available right here in the forums.