This autograph is a very poor fake.
Like so many on Ebay. To prove a point to someone recently I searched on 'David Bowie signed' on Ebay and out of about 30 auctions only 2 or 3 looked authentic.
Bowie has himself said that Ebay is full of fakes.

Basically I would never buy any autograph off Ebay. Most say COA but what use is that? It's only the word of the seller after all!

If you want genuine autographed stuff why not check out Bowieart.com - it's a little pricey but some things like the utside cover art are reasonably priced and the signature is 100% genuine. Or pop over to bowiestore.com where they have a few signed ziggy prints left.