The best way is that you can find a really popular one and then develop it.With enough pr and traffic,it'll very easy for you to get a good price.
if the domain name is popular people will contact you soon if you in hurry to sell then look in BST section in DP to list for sale also u can try in namepros.com , bido.com , dnforum.com
Two different questions really. As for selling... find a desirable domain and put relevant content on it. Get your visitors up and try and raise your PR ranking. The put it up for sale in places like this forum, namepros, sedo, dnforum, etc. You can also use those sites to buy domains.
I think if the domain name is popular, it's already registered by others Most time, you need to develop the domain's value, which is not absolute.