Comes down to individual preference as to what you want.
Yeah, you could use an old engine/trans out of an Explorer and cobb it togather just like many here have done.
Or you could do it the way he did, new stuff, professionally done. Just a matter of what you want.
He probably spent twice as much as he's asking, so he's not going to get his money back. But if you were going to have a swap like that done, by a pro, then it would cost a lot more than the 13 grand he's asking.
Not saying you can't do it for a lot less, but just price out new equipment that he listed, and it's way more than the 13 grand, not including the truck itself.
Since I'm not capable of doing a swap like that, I would be left with 2 choices, spending money for a swap using used parts or one with new...I'd go for new, but that's just me.
Also, he's taking offers which means he will take less than the asking price.
No right or wrong, just personal choice.