There are positives and negatives to buying vs starting your own online business.
One thing I would be wary of is paying the right price if you do choose to purchase an existing site - I would say that from experience, 2x or 3x gross profit and a contribution towards existing customers or traffic (commonly known as good will). I once was approached by someone selling a website that turned out to be nothing more than a fancy homepage and a doman name for £15k!
Best,
Andrew
Founder of www.urbanalpha.co.uk - An online designer men's clothing retailer