"I'm looking for a way to make experience you did not train yourself not as appealing or valuable."
Just to preface I'm a newer player who does not trade for exp.
It'd be nice to not negatively affect people who do trade for exp too greatly as I'm sure they spend some real money and keep our DP economy in check. But, I also feel like it would be nice to reward the efforts of people who go on vortex and train pokemon for themselves too.
My idea is to create a separate statistic for self-trained exp and have it added to the global trainers formula, and prominently displayed on their profile to show off. (and if you wanted to take it even further, to the poke's profiles as well).
Obviously it can't be backdated, but the profile stat could be implemented and start at 0 from the day it is to give everyone a fair start. People like showing off stats and I feel people would like to train a lot to get the "self trained exp" on their profile high.
If it were added to the formula for global top trainer, it could be worth (example) 1.5% more points than exp that was traded to them. If the exp leaves the account, it changes to traded exp in terms of global points.
So, we don't kill exp trades and trainers. We also reward people who train for themselves and give a fair chance at progressing through the ranks via training yourself and not trading. We can also see how much a person has trained right there on their profile - an added bonus being that people will know who to trust more easily in terms of legitimacy. Hypothetically I'd be more comfortable dealing with a trainer who has trained a lot, rather than somebody who has possibly been banned recently and doesn't have much self-trained exp displayed on their profile.