Keep in mind that "asking price" on ebay does not mean they go for that much, its like Beanie Babies, a lot of people ebay list some rare ones for $8000, but if you look at actual sold through ones they only sell for like $15 to $20. Also dont forget you will pay to both ebay and paypal part of your sale price. Which is about 10% total. So if you sell one for $500, you will pay about $50 in fees. plus shipping costs. If you consider the fact that you also paid tax on that $400 Target purchase (and actually paid around $435 for the console), then you are lucky if you break even.
PS4 Pros on 4.55 usually go for close to the retail price since its so easy to get them. Often they will actually go for less than retail especially if box is opened. Its still very easy to get 4.55 Pros even in one of the biggest cities in USA. Targets, GameStops all sell 4.55 Pros still. Someone just posted on reddit yesterday that their local GameStop has over 15 PS4 Pros on 4.55.
The whole situation is not helped by the fact that there is little to no benefit of owning a Pro since very few PS4 Pro supported games run on 4.55 or earlier, and those that do have very tiny differences (one could argue not worth extra $100). Uncharted 4 for example, or Horizon, ud have to look pretty hard to see Pro upgrades.
And slims run cooler and quieter than the Pros (and probably will end up lasting longer because of it) and are even easier to get (since they came in bundles and you can guarantee firmware by buying one).
On top of that, we know there is a 5.05 exploit in the private. So with time basically any PS4 console manufactured prior to March 2018 will be exploitable. So there is no rush of buying the older 4.55 consoles.
And the world is different today than PS3/360 days. Those days by modding your console you got access to a huge library that would cost a fortune to get otherwise. These days with PS4 Plus a lot of must have titles you end up getting for free. Online is a much bigger part of gaming these days too.. and you kind of give up both of those with modding your PS4.