Firstly, it is Called Gheed's Charm like the annoying act 1 merchant.
Secondly: It is a grand charm that gives you Magic Find, Gold Find and as a special reduces the prices of buying things from vendors.
Thirdly: Blizzard has put a restriction on it so you can only carry one at a time. Unfortunately this restriction also makes it impossible to put it in a trade window.
Fourthly: On paper it looks like a nice trade, but beware! The majority of trades involving Gheed's Charm or Annihilus are scams! You should only initiate a trade requiring dropped items with people you trust 99-100%. Unfortunately most Gheed's traders are likely trying to rip you off.
Conclusion: Gheed's Charm is awesome, but it is not worth the risk losing a Trang's Armor for nothing. So, don't do it!