I personally like the way James from Thrillhouse sold stuff, whatever he paid, he increased the price a certain amount (I don't know the specifics). This caused me to find weird pricing situations such as Sega Saturns ranging from 30-60 dollars. There was a Sonic the Hedgehog complete going for less than loose. This is because he personally paid less for the complete one. This created weird situations. But allowed some sweet deals to go to the shopper. I found some gems for ridiculous prices. There is nothing immoral about that. In fact, it is weird for a storeowner who has to keep the lights on. Sometimes I felt bad as the buyer if it was a good deal... But I always consider him to be the top notch of all game sellers.
I try to do this in my own trades. If I pick up doubles essentially for free and someone wants them, I try to just send it to them. I want to pass on the deal that I got. If I found Earthbound for $5, I would get a kick out of selling it for $8. You made a pretty good profit and made someone's day. You also hopefully helped reverse the artificial climb that the game has taken. This may sound like BS. But a few people here can attest to me passing along sweet deals.
Basically, I would promote the method of remembering how much you paid. I don't see a problem reselling. But I do see a bit of a problem with buying Earthbound for $5 and selling it for $100. Sure, someone's willing to pay. But it is artificial. Its like how Soda costs $9 at a sports arena. There is a certain amount of immorality in such a huge profit margin in an artificial market.
I have no problem with resellers. But I do have a problem with how far it can go. I used to collect Nintendo pretty intensely in the 90s. But the scene changed. Nosferatu selling for $40 is wrong. Earthbound selling for hundreds is wrong. Certain unnamed people who buy several copies of Surprise at Dinosaur Peak or Duck Tales 2 to watch the price go up is wrong. I left the scene and am glad. But I'm a grumpy old man.