difficult really as as buyer you could get shafted what ever method you use.
don't do paypal gift use paypal services or goods and you should be covered but read the small print really carefully.
Bank transfer but no way for you to get your money bacj
Is there no way you can meet up and do it old fashioned with cash, it's the only safe way. If the person really wants to sell there bike they will do all they can to get your cash.
Just be carefull with your hard earned cash
What if the bike is not as described are you willing to take the risk?
things can be hidden/adjusted in photos remember