If you have a domain name that someone else is asking to buy, that is hint 2 that it might be worth something.

Hint 1 was the fact that you registered it in the first place.

I don't think there are any hard and fast rules here, common sense (and lots of web searches) should tell you if a particular name is valuable.

You also have to factor in your own personal values. Let's say you own "llamameat.com". If you just bought it hoping someone might want that name for a porn site, it is probably worth a couple of hundred bucks to let it go when "animallover@kinky.net" asks about it.

However, if you are a militant vegitarian, you might want to hang on to it, no matter the price offered, if you think "carnivor@jerky.com" just wants that name to expand his exoctic snack food procuct line.

So, the answer is, "it depends".

Some sites like Estibot.com and Valuate.com use a computer program to estimate a price.