I would be cautious, but still proceed, since it sounds like you'd actually be seeing it in person before paying the seller? Just a couple thoughts:
1. What's his ebay feedback (specifically as a seller which you can see be drilling down on the feedback score)? It's not a sure-fire way to figure out a scammer but if he has tons of feedback (and a history of selling Mac hardware) then I'd be less wary.
2. Ebay charges a hefty fee (up to 10% of the sale price) and PayPal takes another 2-3%. I'm not sure what marketplace his $700 listing is on but maybe it takes a much smaller cut. I agree $700 is way below what it's worth, but he may have seen it not selling at $2000 and would rather have some cash now rather than wait for someone to pay more.
3. IIRC the biggest issue with the nMPs were the video cards failing. Apple has a recall/repair program for the affected ones though:
https://9to5mac.com/2016/02/06/apple-mac-pro-repair-program-graphics-video/. Would you actually get the opportunity to boot it up before paying? You might also ask the seller for the SN so you can check if it's still got AppleCare coverage and/or if it's affected by the video card issue.
Edit: Ugh, just saw that the repair program was set to end two weeks ago (May 30, 2018). If it's affected though you could still see if Apple would make an exception considering the circumstances and how close you are.
Edit again: My understanding is that iCloud Activation Lock on Macs is actually easily defeated (
https://tidbits.com/2016/07/22/disable-find-my-mac-by-resetting-nvram/) IF and ONLY IF a firmware password hasn't been set (which it hasn't for the vast majority of Macs out there). You can test if a firmware password is set by trying to boot from USB or the recovery partition. See more details on that here:
https://support.apple.com/en-us/ht204455
If a firmware password is set and the seller doesn't know what it is, run away. My understanding is the only way to fix that is to take it to the Apple store where they will ask you for proof of purchase.