The act of negotiation itself is a cost/fee that people would rather avoid - and for small purchases the need to negotiate is more costly than the actual product price.

Lack of negotiation isn't a curiosity, it reduces friction of transactions - see item, point at it, give the money, you're done; which saves effort for both parties.

Fair enough. But even in negotiated transactions, the seller sets the final price that they will or won't accept.

In a fixed market the fixed price still remains the upper bound for the negotiation. [Until there are more demands outside of the predetermined terms]

Exactly like the buyer.