A few points when buying motorcycles for restoration.

1. Registration Document.  This is important but not vital, wherever possible I try to choose bikes that comes with a V5 registration document.  This is a good indication that the person actually owns the bike.  Buying a bike without a V5 means that you do not have the reassurance that you are buying from the registered keeper.  Ask for any old MOT certificated and bill etc, these will give reassurance the deal is legit.
   
2. Transportation.  Most motor bikes are too large to fit into a car whole, so a van or trailer will be required.  Unless you are prepared to strip the bike down and chuck it in the boot.  I have done this with various bikes using a 306 and a sierra and not had too much problem.  Best check that the seller will not object to you doing this on their driveway though.
3. YOU GET WHAT YOU PAY FOR.  If you pay £20 for a bike, don't expect it to be a minter.  Likewise if somebody is selling a newish bike very cheep, make sure that you aren't getting 'hot' goods.  The police will confiscate stolen property regardless if you knew or not.
4. Be honest with the seller and they should be honest with you.  I would rather know if the engine has a bit of a knock, so that I can do something about it rather than letting it blow up.
5. Bike spares.  Ebay is great for spares, but again you only get what you pay for and somebody else's idea of 'in good condition' may not be yours.  It helps if you know your stuff and never be afraid to ask for a more detailed explanation or more photos.

Just a few points about buying bikes for restoration though ebay, I have done several this way and made good on each bike.

Cheers

Phil