Even Bowie himself has said that the vast majority of 'signed' stuff on Ebay is fake. If you really want a 100% genuine autograph then why not save your pennies - go to www.bowieart.com - and get a 100% genuine signed 1.outside cover artwork for $150 0r an 'Iman' signed by Bowie and Iman for $125. Both are 100% genuine.
I don't want to disappoint you but the autograph you bought may well be fake. I'd be more inclined to think it was genuine if it were a signed 'Heathen' and not an ESH as that was what most of last year's public signings insisted on. No-one really knows do they! That's why you should save up and get a genuine article rather than feeding an Ebay potential-sheyster! A certificate of authenticity is worth nowt really!