There is a good chance that it is all perfectly legit, and if you are really in the market for that GPU, then what harm is there in meeting to see the item? Just do all the research you need to to help determine legitimacy of the item, such as checking the serial number through the window on the box, checking the seals, etc. And of course meet in a well lit and traveled location for safety.
I have been selling new and sealed video games and video game accessories through Craigslist and other similar sites for a few years now, and selling them for a fraction of what the used copies would cost at GameStop. I know that I miss out on a lot of sales because people probably think I am trying to rip them off, but the games were $3 - $6 each at Kmart because they don't mark stuff down or have good sales, then when the items are marked down, it is only in the computer, so no-one knows the games are cheaper and don't buy them...causing more markdowns.
Sorry I digress, but I am just making the point that the person could have gotten into a situation similar to that. Or maybe they got it on a good sale for the games and thought to sell the GPU itself or planned on using it, but changed their mind. Of course if you meet the person and don't see it is the correct item you thought it was or not 100% sure it is trustworthy, just walk away. No harm, no foul.
And I agree, I definitely prefer EVGA. Yes he could seal it himself but typically that plastic sealing will be a different quality and type than the factory one. Look up the unboxing videos and such to tell a difference.