It currently has website traffic to it.
Website, Blog and Email setup and all access details given immediately on purchase.
Time can be spent on getting more website visitors as the website is already built and optimised.
Domain may have a Google pagerank.
Branding / Logos already created.
The website has ‘age’ which is a factor in Google’s ranking factors.
Full of relevant / unique content.
Existing Links to it (hyperlinks).
Generally there is more chance of the website being a success than starting a brand new website.
The website is operational and ‘up and running’ already.
Has an existing client base.
The previous website owners will lend support.