Engineers always over-specify service intervals on the short side of reality. German engineers take this practice to a ludicrous extreme. There are less polite ways to put it, to do with covering their sitzplatz. Herr Rohloff, a great German engineer, was an enthusiastic practitioner of that norm.
In my opinion, take the ride around block as already advised, and if there is no audible or tactile problem, and on the assumption that the owner's asking price is reasonable, tell him you'll take the bike if he'll do the service. It saves you a few bucks and, if this is your first Rohloff, you'll see how it is done.
What I mean by audibly and tangibly is that you should be able to feel and/or hear if there is something wrong with the box, even on a ride around the block, or a few blocks. A good box has 13 clicks between the gears and no graunches and isn't excessively stiff. It takes many thousands of miles to run in a Rohloff, so a little stiffness in the change is nothing to worry about -- on the contrary, it is probably a sign that the box was little used, a factor which in most cases will be borne out by the obvious cosmetic condition of the box. Or ask the seller how many thousands of miles on the box. I declared my Rohloff run in at about 8000km or 5000 miles, but several years later it is still getting smoother. (Contrast two Shimano Nexus Premium boxes I wrecked with less than 5000 miles between the two of them to get a relative idea of the mechanical strength of the Rohloff.)
You've already been given expert tips on what the oil coming out of the box should look like. Those tips assume a well-used bike. There is a small danger that on a newish Rohloff there are still bits of metal teeth being knocked off, a natural process, and that they will feel slightly gritty if rubbed between your finger, and that you might mistake this for damage being done when instead it is a sign that you're getting a little-used bargain.
Good luck.