I didn't think there would be but the guy saying his system was more optimized to the 8700k I thought I should check first.
His statement sounds suspicious. I can think of two reasons why a system would be better optimized for an i7-8700K rather than a 9900K, and neither are very good reasons to trade.
1.) Not enough power from the PSU for the 9900K. But given that scenario, he probably shouldn't be using an i7-8700K either.
2.) Save money. Even with this, swapping it out after the fact does not save money.
There is the assumption he has a slower GPU than he thinks he should have, but even at that he'd still be losing money on the trade for practically no gain and a small loss in performance. I suppose there is the possibility that you are giving him a reasonable sum of money for the trade, in which case he might just want that money.