Any engine fluid leaking is not normal, especially oil. That should never be considered something you can live with, unless the car has a chronic problem. In that case, I wouldn't consider that car for purchase.
The particular DB9 you are considering, is very low miles. While that is indeed an inducing buying point, my first reaction to why the seal is leaking is because it hasn't been driven enough. Those seals dry out, and the lifespan of them are shortened because it's not getting lubed on a regular basis.
Personally, I'd be offering a few thousand less for that car with that problem, unless the current owner wants to take care of it before selling it.
Aston Martin cars get such bad rap for reliability and such and a lot of problems could be avoided if their cars were driven a little more often. 1-2K a year and driving it once a week or a few times in a month will do wonders for keeping it out of the maintenance shop.
I see the 2006 model you are considering is a Volante. If a convertible is the only model DB9 you want, your choices expand greatly if you're conducting a nationwide search...I wanted a Coupe, which is much harder to find a particular one with the right combination. You'll have a much easier time finding one you like.
8K is a chunk of change for a repair, especially buying something that you obviously are excited and enthusiastic about. If that's something you're willing to fix yourself, then go for it.
You want your Aston Martin experience to start off on a positive note and be able to enjoy it, relatively problem free.
Best regards,
Thomas M.