I boght my Samsung laptop 2 years ago refurbished and when I registerd the warranty it registerd as brand new. Turns out some of these companys get new stock from chain stores and this gives the company like Samsung a outlet for the previous years stock.
This isn't making any sense, it's very easy to find new computers from the major brands at prices below $500. Check some deal sites, to find the sale prices. You can also get refurbs from Dell et al, but they also go on sales that the deal sites will cover.
There's no real reason to deal with random 3rd parties on eBay when your budget is enough to get something from a major brand instead of some old $100 special built in someone's basement.
No I wouldn't buy a refurbished computer on eBay. There are stores such as Doublee Computers in Bend, OR - who specialise in refurbishing computers and they will also offer you a guarantee so you have security if anything goes wrong. I know this company have refurbished laptops starting at around $150 so worth a look.
Many of the big players like HP and Dell have outlet sites where you can get scratch and dent or refurbished products for very similar prices. The only REAL value to ebay is if you can get an used part in a pure auction or if you need to find something out of production.
That being said, I buy refurb stuff all the time from ebay and have had no problems, but its usually the stuff that I can't find on the factory outlet sites, I always start with those before turning to ebay, because MOST fixed price sellers have their prices set at what new prices would be anyway so its hard to find REAL value on ebay... -_-