Lately I've been considering buying an existing site - basically I don't have to deal with choosing a domain, writing the basic content and basic SEO. On the other hand, it does often cost quite a bit. What is your experience of buying existing sites?

It depends on what type of website you want to make.
Some sites are harder to find than others but you could always buy a domain with good traffic.

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well buy an existing forum will be a hit idea, developing a forum takes a bit time, needs more efforts.

i think it is good if you will keep the same major of the one that you are buying. but if you will change the major i prefer to make new one.

Buying a website can be better or worse then starting your own. It just depends the way and what site you want.

Depending on the niche. I wouldnt recommend buying a proxy site though because it may already be blocked.