Hi there Owen,
Where big name signatures are concerned, for peace of mind, My advise is buy something that has a certificate of Authenticity. Buying signatures like The Dons is something that you shouldn't do half arsed. Do it properly. As time goes on and the item is handed down through the generations, you will always have proof that the signature is real and the item should increase in price. Whereas a signature that doesn't have a COA will be more prone to be questioned as unauthentic.
I do I.P. autographs for myself and anyone who wants to trade with me with no COA. Believe me it takes along time to build up a good reputation where people generally know that if I trade with them, the item is hand signed and authentic. I trust the guys I trade with on OCT but there is always that very very small % of doubt (which comes down to human nature) and I think guys like Girds and Rekless who I swap with regularly will say the exact same thing.
When you are collecting for fun like me, COAs are not so important. When collecting a family heirloom to be passed on, purchase an item that comes with a COA
Hope this helps
Cheers
Dennis/Blade