But how do you contact the seller? The only way I know is the very public "Ask the seller a question", which I'm very reluctant to use when trying to haggle, it's rather unfair on the seller to do that, I feel.
I mean, how would you like it if one of your listings acquired a public "Hi, I think your jewels are overpriced, but I'll offer you 50%, ok?".
I hasten to add that I've not checked your listings, I'm only guessing you sell jewels, and I'm not in the market for them!
But I agree with your general principle. On another site, only today, I quietly sent a message to someone in my town with inflated ideas of what a pre-owned wheelbarrow with a puncture is worth and wrote "Hi, if you don't get any bids I'll offer a fiver!".