Website Investing
It's not easy! Keep in mind that I am looking for low cost sites which I think have potential. I am not looking for premium domains with proven profitability. I spend well under a thousand dollars for any given site or collection of websites. For me, it's easier to build my own new sites than spend money to buy them. I have old sites from which I can send links to new ones to help get them indexed. There is no real benefit to spending a lot of money.
However, your circumstances may be different. Buying a site where you can add significant content but could not build yourself could be a good investment. This would be especially true of a forum or blog.
I have had my best luck buying sites at digitalpoint.com or sitepoint.com. I look on Ebay occasionally, but you have wade through so much garbage it gets discouraging. I know people buy on sedo but those sites are out of my financial range.
These are small marketplaces with limited offerings. I am sure that as the profit potential of websites becomes more commonly understood the number of venues where websites are bought and sold will increase. Right now it is mostly "Techies" who can play the gaime.