eBay is the bottom of the market, where all the dodgy, ill-maintained, incomplete, spares/repair, piles of scrap, etc cars end up when they won't sell anywhere else where transactions take place in the traditional way - ie a seller indicates a price ono, a prospective buyer goes and sees the car then finds a few faults, giving him leeway to offer slightly less than asked. By making "scrap" national and removing the cost-effective ability to view before purchase, they've created an entirely new way of offloading junk.
Occasionally, a decent car comes up too.
I've bought cars a few times off eBay, in the <£500 bracket, but also let a lot of them slip away due to excess cost. Just factor in a "discount to not view" into your estimate of value and luck should average itself out over time, if you buy a duffer or not. I've personally NEVER bought a car with no V5, its not complicated to register it in your name, takes a couple of weeks at the most to arrive, so if its not there its either sold on quickly (why??? cos its scrap?) or dodgy in another way. If the seller can't cope with filling in a form for the V5, has he coped with regular maintenance and checking the oil occasionally?