Anyone having experience buying computers on Ebay?

First off that is in Denmark I believe (.DE) and unless you were in Europe itself, you may run into many issues (least of which is the damn taxations added onto that purchase). Best buy and other Big Box store use Ebay to get rid of old legacy stock, people get rid of systems they can't use, and some cheapo makers (China) sell on there. It is best to always go with a highly rated person, with a positive track record, but it is hit and miss otherwise. PayPal / Ebay guarantee the purchase by being the middle man and hold the money till you receive delivery AND are satisfied with the product, or work out how to send it back.

Can you sue JohnEbaySeller like you can Best Buy ? No not really, it isn't worth the effort and expense trying to track the person down, but with a normal store they are under many regulatory and compliance laws that make them very wary of someone getting a lawyer to resolve the 'issue' which can leave them losing MILLIONS of dollars even for a $1 purchase (just look at the infamous Hot Coffee from McDonalds lawsuit).