When one buys a 4-5 years old machine, they don't usually expect it to match the specs of the current ones. Such buyers are rarely Apple enthusiasts or professionals with high performance demands. Such machines are usually bought when the budget is really tight, when performance does not matter or when the buyer wants some kind of trial with macOS and isn't ready to pay the full price Apple asks for their current models.
Quite likely a lot of these buyers will be shopping for a '13 inch Macbook Pro from 2016' and will be comparing your offer against other '13 inch Macbook Pros from 2016', not against the 2020 specs. Because if they were ready to pay more for performance, they wouldn't be looking for a 4-5 year old machine to start with.
And I don't think 8GB will be 'too few' for a non-demanding consumer 4 years from now on, to be honest. Past 4 years did not create such a shift at least. I've been helping a friend to pick a laptop in the price category that felt acceptable for a non-enthusiast (~500$) and most laptops for 'normal people' like her still come equipped with 4GB. And it's actually enough for what those people do with their laptops.
That means 8GB that my base 2012 rMBP15 came with is still overkill for a lot of people. A lot of them are buying brand new laptops with as much RAM on board as my White Macbook had in 2007, which was almost 10 years ago. And the desktop/laptop technology development has only slowed down since then, I'd say.
With this said, I don't actually advocate against buying more RAM. I think it makes sense if one wants to enjoy using their machine for as long as possible. I just doubt it will save money resale wise. Yes, it will sell for more, but it will originally cost more too, the sell may prove to be harder and the % of investment lost may end up being a little higher than for a base machine. Doesn't mean it's not worth it though. We buy computers to use them ourselves after all.