Namecheap did not screw you over. Namecheap is just the middleman. They can't compel the seller to sell you the domain at the clearly erroneous price of $31. You tried to take advantage of a mistake in price and it didn't work out, and now you're trying to use HN to pressure Namecheap into honoring something that was clearly a mistake.

I purchased the domain at $31.

This is false. You made an offer to purchase a domain for $31 that had been listed before, and after, for over 100x that price. And clearly, the mistake was that the seller did not intend to list it on the Buy Now portion of NameCheap's domain marketplace for $31 when it was previously listed for $3900 (and for more after the mistake). The reason I bring up the courts in my earlier comment is that legally, NameCheap is not compelled to honor the $31 mistake price, nor is the actual seller of the domain. Nor are they morally obligated to honor the mistaken price.

Do you think the seller would have jacked the price up to $8000 had Namecheap not revealed my interest and intent in buying?

No, you tipped your extreme interest in buying the domain when you said you were still interested in buying it despite the 125x increase in price. This told the seller that you were price-insensitive and that they could raise the price substantially without affecting your interest. That's on you.

I used some history tools to figure out the domain that you were interested in and it's not worth it. Seriously. There are better domains for you out there for less. Don't get emotionally invested in the "perfect" domain name. Remember that Facebook was originally "thefacebook.com" and that didn't stop it.

They, at the bare minimum, owe me a refund of the $31 I paid two weeks ago for the domain they still haven't delivered to me.

No, the $31 refund is the only thing they owe you.