Welcome to the Forum
Some advice....well ! Youve already discovered the Forum at the Aerodrome so that is good.
Honestly, if you know very little and you want to get one of the big names like MvR,
LvR,Boelcke or Immelmann in signature form them please forget ebay until you have put in some study into the field. And definately until you know otherwise follow the maxim "If in doubt chuck it out !"
Certificates of authenticity , lifetime guarantees ,should be treated with a pinch of salt.
Low starting bids or 'private bidding' appearing in the auction should in general raise alarms. Nobody is going to run the risk of letting a real MvR go for $50 and quite often the private bidding is only to stop experts contacting the bidders to warn them off. (Of course this is not true of every situation and some use the private bidding to get rid of interfering non-experts!)
Use the escrow service that ebay lists if you are going to part with big cash. They have licensed middlemen to hold the payment until you are satisfied the item is genuine.
Never send cash..ebay does not protect deals done in cash.
My best advice is save up and fork out the 4 figures required for a big name item and buy from DerRittmeister in USA or Stefan Korlin in Germany or BP stamps in UK (as and when he gets the occassional WW1 item).All absolutely genuine expert dealers who would never sell an item unless they are absolutely convinced it is genuine. The old story..if you want the best (in this case as unquestionably genuine an autograph as one can ever likely to be ) then the drawback is you unfortunately have to pay for it !
If going on your own you need a library of sample examples gleaned from every book or magazine you can find, a 200x microscope to detect items that are printed , learn about flow,size, rhythm, ink density, crossovers, and what else ? Im not sure , Im still learning!
Good luck
Ehrenpries