Especially as the world ages, most barter stands offering trades I would want to transact are full - but the only way to find this out once you discover them is to make the journey there, usually loaded to the brim.
This is a huge issue for newer players especially, who depend upon trade with more veteran players; they tend to be selling bulk low-tier items like bricks and whatnot, and avoiding wasted trips (deadheading) would be a huge QoL improvement.
In addition, tools like the Community Fair market stall report could adapt to this signal and stop reporting all the stalls that are full.
A basic check of the destination inventory to ensure that it can fit another click of the trade button seems relatively light on the server - it would only need to be performed when a trade was performed (Can I do this again?) and when the container holding the hand was interacted with.
Perhaps resetting the flag could simply be a matter of placing the hand again, forcing a recheck. Emptying the desto container completely, then putting the hand back, seems a light enough workload on the seller.