It would very much depend on what type of business, the size of the website, what niche they operate in, how is the site performing etc etc. I have bought a few sites myself but my approach is very different. Like with any type of purchase, I look for distressed assets, in a niche that I understand very well. I use various metrics/tools to assess a website (too numerous to list here) and I look for sites that have been under performing. I look for things like, drop in the SERP results, loss of Keywords, etc etc. Once I do my analysis and if I deem that the site has potential and it is salvageable I get in touch with the owner and make him an offer. More times than not, I succeed in buying, Once bought, I have the SEO and Technical skills to bring the site back to profitability. I usually sell a share a remain as a silent partner.
Having said that, buying a website is very different to buying any other type of business. You are at the mercy of google (for search results) and they can change. The other point is this, if a site is designed properly and the SEO is implemented correctly, it can continue running with minimal effort (generally, not always). Why would anyone sell a business that is profitable and established? More often that not, it is because something is wrong with the business and they are hoping that the buyer will not notice. But not always. My most profitable investment was in a very niche, well established website. The site had started to lose rankings (but was a very well established site) and I head that the owner was selling it. I contacted him, fully expecting this to be some scam of sorts but it turns out he was just exhausted, and wanted out. It did not help that his SEO knowledge was 5 years out of date. I bought the site, spent some time and money on it and withing 3 months it returned to profitability.
Long and short of it is. Marketplaces are not necessarily the best place to find value. You have to understand the niche/market that the site operates in before you buy a site. You have to do your due diligence very thoroughly. You have to be prepared that google might change its algorithm and that it will go down the drain. All in all, can be profitable if done right but also chances of failure are quite high.
