There are ways to detect a cancelled trade versus a player lagging out. Also "accidentally grabbed table" ya lol... Could just have a confirmation if you truly want to trade. If these are meant to be trade tables, not sales kiosks then you need to set circumstances in which discourages players from just occupying them and encourages them to actually organize and complete trades.

Not saying the idea is completely perfect, but in my opinion it's a step in the right direction to fixing these apparent problems.