For me it's a side business, nowhere near full time. If you have the right domains it could be full time, but I don't think there are many full time pure domain sellers. Even the big guys run registries, get a piece of a business with a domain sale (Candy.com), have websites (PalmSprings.com), or generate good income from domain blogging.
The bulk of the offers on my domains come in at $50 to <$1000. Most people want to pay that range, no matter what the domain is or how perfectly it suits their needs. Whether you accept an offer depends on a lot of things, do you need the money, do you feel the domain is really worth more, etc.. One general rule I have for myself is always counter offer with a higher price.
If you have the time I think it's valuable to learn to build webpages and sites, even basic ones. There are lots of site builders out there that are easy to use to build small to medium size information websites, for example. Blogger.com and Wordpress are easy ways to start blogs or can even be used for websites. Pick a topic you are interested in that way you will more likely stay with it. It also helps if your topic will appeal to a large audience or an intense niche audience, and is a topic that is somewhat commercial (so you can generate income with something like Adsense). Also, if you can put your energy into one project and do really well at it, as opposed to starting up many different websites, that is also a good idea, though hard to do.