To my knowledge, Ducati never made an 899 Corse, so it's likely a custom paint job, with or without any actual upgrades. Tell the seller you never heard of an 899 Corse, and ask what exactly is different from the base model. If the seller is the original owner, ask for anything (manuals, invoice) that would qualify the bike as a special edition. If it's not the original owner, maybe the guy got the bike like this, and really believes he has a Corse...
In any case, if the bike is in good shape and the paint job is well done... why not? I personally made a sticker kit for my 899, inspired to the 1199R, even if there is no such a thing as an 899R...
By the way, it's "Corse", not "Course". "Corse" literally means "Races" in English, but it'd be better translated to "Racing", as in "Ducati Racing".