The real goal may be to identify suckers
Con games on the Internet are getting very, very smart. There's a trick: Screening victims to exclude the savvy. They want the right ratio of greedy, naïve and crooked... and they don't want anybody else.
Microsoft Research wrote an amazing paper about how scammers deliberately mention "Nigeria" to repel the savvy: leaving only the naïve. Offering to buy aged Amazon accounts is obviously shady as all getout, and so it also selects for the crooked, and the greedy.
So the aged Amazon account may only be a bonus; the real goal would be to identify someone who can be manipulated. You might get all the usual dodgy scam stuff. Transferring too much money "Oh, could you transfer the extra back please [using a totally different and non-reversible method]? My father just went in the hospital." (and then, the payment reverses). Printing fake paper checks using the account info, and using those against other scam victims. (Multi-way scams are now a HUGE thing, where they play 2-3 victims against each other).
Or, since you have flexible morals, they may offer you an "office worker" job cashing checks. (those same checks I just mentioned, but other people's).
Or they plan fraud with your Amazon account
You are not allowed to sell your Amazon account. Ever. It contains your reputation, both publicly and internally in their risk-analysis system.
Amazon actually contains an "eBay" inside it - anyone with an Amazon account can list items on Amazon Marketplace, which are inter-mixed with regular Amazon results. ("Sold by and ships from XXX" right under the Buy Now button). Further, anyone who sells on Amazon Marketplace can place the products in the Amazon warehouses and then, those items ship with Prime. ("Sold by XXX and Fulfilled by Amazon"). There is so much mischief a fraudulent seller can get up to, using the trust Amazon has placed in you.
It's not just that it's an aged account. IT's also a US/CA/UK account. As you surmised, these people are foreigners, and Amazon has done the "Know Your Customer" diligence to ascertain that you are an American/Canadian/Brit, and as such, have a level of trust since you're well within the reach of US law enforcement if you were to commit fraud.
If you're very, very lucky, the worst they'll do is buy a bunch of their own products with your account so they can leave fake reviews - that arms race has been going on for years. (to show the extremes they'll go to, search the web for stories of people receiving unordered product. They're willing to give up a lot of cheap Cheese junk to buy some good reviews on the platform).
If you're very unlucky, they'll use it to fund terror.
Whether or not you are OK with them doing the above, the appearance will be that you are OK with it. Since you conspired to do it.
You are implicated in whatever they do
Or, don't leave your ID at a crime scene...
Both the authorities and a sufficiently motivated private party, if burned or concerned by activities with your former account, can chase you down. For instance, I would file a "John Doe" lawsuit, subpoena from Amazon the identity data of the account owner, and your name would pop up as a past owner. Discovery would have a fair chance of turning up the fact that you sold your Amazon account for money. It goes downhill from there. The argument would soon come up that you knew, or reasonably should have known, that selling an Amazon account was a breach of contract, and that the buyers would likely be up to no good. That would remove any defense of innocence, and would increase your risk of liability, even making it hard to wipe it out in bankruptcy.
Such legal action tends to be full of uncertainties and might make for a rather nerve-wracking year, possibly leading to a substantial cash payout, at the very least for your own lawyer.
While the chance is not large, the risk is certainly large. So unless you have a strategy for dealing with those risks, you might want to think twice.