This have nothing to do with losing the tent or keeping cuz every player has his own amount of charges even if you have 1 charge left and you sell the tent and pk later another one you still keep that 1 charge on your tent. My suggestion is just that you can reset the charges manually in change to lose the tent whip, its annoying for players to keep track when they got low amount of charges left and have to kill the charges on a npc or revert in combat situation.
About the sanfew it should cost pkp since it cures antipoison but it shouldnt cure venom at all cuz it take's a inventory spot, some people have a serpentine helm to stay immume but also to safe a inventory space, those small difference should be added cuz its a risk you take with you so small adve tage on the place would be nicely rewarded.