Many people have brought up suggestions to fix the inconvenience of multiplayer shopping. I think you have some good ideas here. There is also currently a mod for DE that "might" be what you are looking for (i plan to try it on next play).
I liked your tagging idea - as it might make more roleplay sense. I'd take it one step further and suggest if player #1 was at a vendor and tagging items - player #2 (the barterer) could click on player #1's portrait and see "view tagged" and the vendor window would pop up for player #2 to purchase those items for player #1.
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Others have simply suggested a options setting (share highest bartering and reputation) checkbox. Purists can ignore the option and those of us that care more about convenience could select box and there wouldn't need to be addition conversations on this topic.
