I'll give you my dos centavos. First of all, it's good that the bike belongs to a member, probably the longer they have been here the better. I have been here for almost two years and I have seen nothing but good things from members of this site.
First and foremost I would want to know if the owner has clear title, in hand. If the bank has the title it can sometimes be hassle (not saying it wouldn't be worth it just letting you know and be prepared). If so the is going to have to be somewhat a level of trust on both sides as well as some give and take to get the transaction complete. If the bank has the title, they usually won't release it until the loan is satisfied (almost never) and most buyers want the title at the same time they receive the bike and hand over the funds. That would be next to impossible if you are in NYC and the bike is in Arkansas. Just something to think about.
I would start by asking him to post detailed videos to youtube. I would want to see 360 degree video of the BDM in real time. I would want to see the bike start, idle, and driven up and down the street. I would want to see the VIN in the video. I would also request that the bike be taken to an independent mechanic that would change the oil and perform an inspection, after which I could discuss with him the condition of the bike. make sure you bounce the VIN off the mechanic to ensure your're talking about the same bike. If the seller is hesitant about the inspection, I would offer to pay for it if when (and if) I purchased the bike he agreed to knock off half of the price for the inspection.
Afterwards, I would weigh all of the information gathered and make a decision on whether or not to purchase the BDM. If I decided to purchase the bike, I would strongly consider taking the extra time, money, and effort to make the trip to get the bike in person. If not possible I would discuss with the seller a very detailed manner in which the transaction would take place. I would want to speak with the bank (if the title is being held) and ask how they would release the title.
Next I would contact shipping companies and vet the hell out of them. I would consider calling my insurance company and explaining the situation to them. The bike should be considered yours at the point of shipment and you should get it insured at which point the shipping company takes possesion of the BDM. Your insurance company may even be able to recommend a shipper. There are shippers out there who specialize in MC shipping and use pre-fab crates made just for MCs, I would definately use one of those.
As I stated earlier, I would discuss with the seller a detailed plan to complete the transaction. If I could not pick up the BDM in person, I would consider retaining an attorney to escrow the 20K until receipt of the bike and title is completely transferred. It probably wouldn't cost much more than a couple of hundred bucks and when you are dealing with 20K it is a very wise investment. You would have to negotiate the terms of the escrow and the release. I would think it would be extreme unfair to the seller to put any more stipulations on the release of the funds except verified delivery of the bike. Once delivery of the bike and receipt by buyer is verified, then the funds can be released.
I know this sounds like a hassle, but losing 20K is a hell of a lot bigger one.
This is just my opinion and others may see it totally different, as I said in the beginning this is just my "dos centavos"
I hope this helps. If I can help you further feel free to let me know, I'll give you my #.