You need a ton of pictures. Then have the VIN run by your local dealer. Now it is time to get serious, figure out how much it will cost to get the bike home. If you have to stop at a dealership for repair will it be on a day they are closed? Will the failed part be something common or will it have to be shipped in. BMW dealers aren't big on stocking parts prior to becoming a dealership. So airheads and older K bike's (bricks) won't have a lot of parts laying around the shop. How many BMW dealerships are on your route home?
The tires, you want to know what manufacture date is on the tire and pictures of a tread wear gauge measuring the remaining tread. It would be nice to know what other brands of bikes wear the same tire in case you have to buy a tire from a non-BMW shop.
How good of a mechanic are you?
How much has the bike been ridden in the previous year? The past three years? I like to see 6,000 miles or so per year.
Did the guy you are buying the bike from do all the work on it himself? If so, when you go to sell the bike you'll have problems. Buyers really like to see BMW shop maintenance records.
Does the bike have expendables with it? Oil and gas filters? Extra oil, electrical bulbs, clutch & throttle cables? Things a rider would keep with them?
Plates? Will the owner let you borrow his plates to get home? Want to chance it and use one of your plates? What are the state laws where you are buying it pertaining to a temporary plate? Will the owner give you a clear title and bill of sale at his expense in writing?
Weather, whether you want to chance riding a bike home in inclement weather. Maybe you could take another route?
Payment, got to make sure the owner will take a personal check, bank draft, postal money orders, paypal, etc. I wouldn't get one money order in the exact amount, but several money orders in case I had to negotiate the price downward.
Deal only with the owner.