AI is already throwing out gifts without any effort on my part -- I thought this was something they did when you were on good terms. So as-is, Human players are benefiting on all sides: price gouging for AI, best deals from AI, all while maintaining positive opinion. To make accepting a lower price actually meaningful, diplomacy would need to be made harder. But then you're tweaking not only trade deals, but also diplomacy AI. All to avoid removing "choice" from the player, which really isn't a choice at all, it's just a bunch of upsides that you can opt out of.
At the very least you'd need the AI to hate you for forcing them to pay full price for an item, and give some way for the player to know where the middle point is so the AI can see the deal as fair. The only way diplomacy works right now is that the AI only takes fair deals; it doesn't use trade opinion as a resource. So maybe a more robust solution would be in that direction? Have some kind of diplomacy currency that can help offset bad deals one way or another. You gain it be giving good deals, and lose it by giving bad deals. That actually sounds really cool, but way out of scope for this change.