Items Eff.=Effect

Tonic: Buy 10 Sell 5 Eff.=Restores 50 HP
Mid Tonic: Buy 100 Sell 50 Eff.=Restores 200 HP
Full Tonic: Buy 700 Sell 350 Eff.=Restores 500 HP
Ether: Buy 800 Sell 400 Eff.=Restores 10 MP
Mid Ether: Buy 2000 Sell 1000 Eff.=Restores 30 MP
Full Ether: Buy 6000 Sell 3000 Eff.=Restores 60 MP
Hyper Ether: Buy 10000 Sell 5000 Eff.=Restores Max MP
Elixir: Buy 0 Sell 10000 Eff.=Full Recovery one person
Mega Elixir: Buy 0 Sell 25000 Eff.=Full Recovery every one in fighting party
Heal: Buy 10 Sell 5 Eff.=Cures adverse conditions
Revive: Buy 200 Sell 100 Eff.=Revives one person who is dead
Shelter: Buy 150 Sell 75 Eff.=On map screen or on a save point is like a Mega Elixir
Power Meal: Buy 0 Sell 1 Eff.=Removes ? and Lock status
Lapis: Buy 0 Sell 250 Eff.=Everyone is restored 200 HP
Barrier: Buy 0 Sell 2500 Eff.=Magic attacks reduced 1/3 on charter used on
Shield: Buy 0 Sell 2500 Eff.=Same as above but with physical
Power Tab: Buy 0 Sell 5 Eff.=Raises power by 1
Magic Tab: Buy 0 Sell 5 Eff.=Raises magic by 1
Speed Tab: Buy 0 Sell 5 Eff.=Raises speed by 1
Petal: Buy 0 Sell 0 Eff.=Used to get weapons and armor in Ioka tribe
Fang: Buy 0 Sell 0 Eff.=Same as above
Horn: Buy 0 Sell 0 Eff.=Same as Petal
Feather: Buy 0 Sell 0 Eff.=Same as Petal
Seed: Buy 0 Sell 0 Eff.=Give this to Doan in trade for the bike key
Bike Key: Buy 0 Sell 0 Eff.=Let's you ride the Speeder Bike
Gate Key: Buy 0 Sell 0 Eff.=Used to open up the Gates
Pendant: Buy 0 Sell 0 Eff.=Opens sealed chest when powered up
Prism Shard: Buy 0 Sell 0 Eff.=Proves Kings innocence
C.Trigger: Buy 0 Sell 0 Eff.=Use with the Clone to bring Crono back
Tools: Buy 0 Sell 0 Eff.=Used to get the Super Charged Masamune
Jerky: Buy 9900 Sell 10000 Eff.=Used to cross Zenan Bridge and to get the Moon Stone back
Dream Stone: Buy 0 Sell 0 Eff.=Used to make the Masamune
Race Log: Buy 0 Sell 0 Eff.=Records your three best times against Johnny
Moon Stone: Buy 0 Sell 0 Eff.=Turns into the SunStone
Sun Stone: Buy 0 Sell 0 Eff.=Used to make Wonder Shot, Sun Shades, Prism Specs, and Rainbow Sword
Ruby Knife: Buy 0 Sell 0 Eff.=Used to short cicuit the Mammon Machine
Yakra Key: Buy 0 Sell 0 Eff.=Used to unlock the real Chancellor
Toma's Pop: Buy 0 Sell 0 Eff.=Used to find the location of the Rainbow Shell

Source The Chrono Trigger Player's Guide