ISO - ISO is just short for "In Search Of"'
FT - For "Trade"
T/O -"Top offer"
Res - "Reserve"=Minimum price a seller will sell for
BIN - "Buy it now"
FG - Forum Gold
Grush - Bugged rush from Normal Act 1 to Hell Act 5.
Bumpers - Someone who is used to get the baal quest in a Grush.
Unid - Unidentified item.
(X)O - X is used to replace a number of skill/life/etc that the user does not know.
OS - # of sockets
HR - High Rune which is the main currency.
GC - Grand Charm.
SC - Small Charm.
LC - Large Charm.
GG - Good game.
GFG - I personally use this to stand for Good fucking game or "God for God"
Upped - Means the item has been upgraded above it's base stats.