I know a few people who create sites to solely sell them, and some of them are awful. You just have to look at the SitePoint Sell Your Turnkey forum.
I agree with what petertdavis was saying, you can see clearly what sites have been developed to keep and later sold and what sites have just been created to make a quick few quid from. The designs are awful and not much effort has been put into the development of the site.
Regarding the evaluation of a website, it can be tough. Most go on the revenue and how much can be made in the long run. HolyLemon.com was up for sale recently, the owner was selling because he was taking up a job offer and didn't have enough time to run the website. The site itself was making over $16,000 a month and was pulling in the visitors (80k uniques). The site probably would have sold in the region of $xxx,xxx, and the new owners know there will always be users to the site because of it's appeal; so they don't mind splashing the cash.
Another way is the domain, I know Lee bought OnlineGames.net a while back, the BIN was $120,000, don't know what the final sale price was, but the site was making around $4,000 a month before expenses. The domain would have been the big factor in the sale, because even though the site is well coded and has a nice collection of games it's not one of the most popular arcade websites. There are plenty of other arcade sites that have five times the traffic and plenty more games that wouldn't go anywhere near the BIN of $120,000.
If you're going to base selling the site on the revenue it makes, usually 10x or 12x the monthly amount is the norm. So if you were making $500 p/m from the site go for somewhere in the region of $5000 to $6000. Then if the domain is valuable, add the amount onto that.
You can base the site on the amount of traffic as well. Many popular MySpace resource sites, such as ProfileJuice.com (which was getting over 70k uniques a day and made $1,300 in three days was sold for around $20,000) have been selling for large amounts.
It's hard trying to explain how to evaluate how much a site is worth, some people evaluate them at enormous amounts, others lower. Just go with what you think, get some reassurance off some webmaster friends in the know and you should be okay.
Sorry if I babbled on a bit, just tried to explain a little.
