As for weapons, isnt it very possible that tnt would release a weapon that does similar things with those weapons and devalue those weapons? similar situation with Ylanas Blasteer and the other weapon (dont even know the name)
Honestly this is just the risk you've got to take, who knows? You could get really lucky and TNT retires the item and you sell before the release of a replacement comes along.
Weapons RARELY ever decrease in value substantially from the release of a new weapon. I think that the Hanso Charisma Charm replacing the Ring of the Lost is the best known exemption to this. The release and market flood of that item has deflated the Ring of the Lost from the 10mil status down to its current 3.5mil.
I personally prefer dealing with HT Battledome Weapons because the prices stay consistent, you don't have to worry about fast deflation, and with some luck the weapons tend to only ever go up in value. See the point and case of the Ghostkersword.