Here are some very useful websites and tools.
For buying and selling websites and/or domains:
Expireddomains.com
Although their primary focus was expired domains (hence the name) they also list websites and domains listed on different domain or website marketplaces.
For example, I also use it to filter websites listed on Flippa.com, as their list of filters is much superior to the one that Flippa provides.
Flippa.com
Probably the most popular marketplace for buying websites, apps, domains and online businesses
Auctions.Godaddy.com
One of the biggest domain marketplaces. I found some gems through their Auctions. I also suggest that you use expiredomains.com for filtering.
EmpireFlippers.com
Although I never used them personally, I have to mention Empire Flippers, another popular marketplace for buying websites and online businesses. They have a thorough vetting process which means they only list projects with confirmed track-record and verified data.
Analyzing websites and domains:
Semrush.com
Great tool for analyzing yours or competitors websites.
I really like their Domain overview for getting a quick overview of organic traffic (traffic estimate, keywords, volumes, positions etc).
Ahrefs.com
Another popular SEO tool.
Their Domain and Page rank gives you a quick indicator about how strong the domain is.
Their backlinks database seems similar to the one Semrush provides, but I find it a bit better.
SEO PowerSuite
I mostly use their rank checker, but other tools also work quite well (Link Assistant, SEO SpyGlass, Website Auditor…).
One downside in comparison to other tools is that you have to install their software locally in order to use it.
Similarweb.com
Great for analyzing popular websites. They provide an estimate of traffic numbers, traffic sources, referrals etc. The traffic numbers are usually only available for websites with at least 1000 visitors per day.