I think if you didn't want to leave negative feed back just leave neutral feedback and explain just the facts as to your experience with this seller, as a fellow seller and buyer on etsy this is what I look for and want my customers to do. neutral feedback doesn't effect their rating (up or down) and they do have the option to 'kiss and make up' if they are really unhappy with a neutral feedback. You've got to be honest, that's what etsy relies on.
You have contacted the seller and if they haven't been able to give you a solution that is good for both of you, then you know how it has left you feeling as a buyer and your feed back does need to reflect this that' s why it is there in the first place so everyone know how your individual experience was.
Maybe ask them if they could do a direct swap, that why neither of you lose out