It always amazes me when people purchase something and then sell them right away and use that excuse "I just can't use them" or "they were too powerful for me and I need to downgrade" Then why did they spend a few thousand to begin with with out researching that stuff before hand?
They could be b-stock and he's reselling them. He could've also got them from someone else for super cheap and reselling them to break even. Too many question marks with vague responses.
If you want to take a chance on them see if you can haggle with the price a bit. Maybe see if he'll go down to $3500.
I have gotten into various projects over the years, bought the parts, then as the project progressed, realized that I needed to go a different direction. So, it happens. Granted, I'm usually talking about $1-200, nowhere near this price point. Also, sometimes people spend big $ then buyer's remorse sets in quickly.
But, on to this particular item.......NEW/SEALED always tends to bother me a bit when buying from someone other than the original vendor! In this case, I personally prefer the condition to at least be "New, opened for testing/inspection and pics only".
That way, at least you are certain that you are buying what you think you are buying, and not buying a box of rocks! It has happened, there are crooks and scammers out there.
Also, I have personally sold an item that was new and sealed, then the buyer came back and said the item was defective out of the box! That taught me my lesson. Perhaps it was defective out of the box, or perhaps the buyer damaged it and claimed it was defective, and I missed my opportunity to verify for myself. Even if the item is new/sealed, I will have to personally test it before selling it so that I am absolutely certain of the condition before doing business.