Yes, if it were stolen or forged it could indeed be returned unpaid.
A bankers draft is essentially a normal cheque, just issued on the bank, rather than the individual. That said, forgery is difficult, and there are extra security measures in place in their issue, which I obviously cannot post here!
If you are in doubt I suggest you take it to the issuing bank (any branch will do) and they will be able to ascertain it's authenticity for you.
As a guide, check that the amount and figures match, that there is no smudging, that the guide numbers have not been tampered with, that there is a crossing sign showing "not more than x", that the signature is written in pen and normally has a reference no underneath (staff no normally) and if it is a large cheque, that there should be two signatures. Unfortunately, different banks will have slightly different procedures so it is difficult to tell!
To make life easier, suggest the buyer pay you by CHAPS or BACS. BACS will take three days, but CHAPS will allow the buyer to pay you the same day, and is irrevocable. For your piece of mind, discounting the goods by the fee (£20 or £23 normally) will probably be worth it.
Hope this helps
MC