Domaining is one of the lowest risk internet ventures you can do. The biggest risk is that you will register a lousy domain, but your loss is limited to the registration fees so the risk is very low.
When I hear a common phrase I check to see if the .com is registered. If it isn't, I register it. Simple.
Of course, not all phrases are worth something. The phrase needs to be in common use, or catchy. Shorter is better. .com only, I would not buy a .net, .org, etc. Avoid hyphenated domains.
I very rarely buy a domain that is already owned by someone else for the simple reason that they are usually overpriced so there is no room for profit on resale.
Selling domains is difficult. Domains are not a liquid investment. You usually must hang on to a domain until the right buyer approaches you, so don't count on fast cash.
Some domains I have for sale include tole.com, WebmasterResources.com, KeywordDensity.com, CelebrityDNA.com, and others.