Whenever something is high in demand and low on supply (more buyers than sellers), the price will rise (it's more rare), if it's low on demand and high on supply then the price will decrease (nobody wants it). After the new worlds come out, people are bound to start pking again, these are items that will very like increase in value after a while.
Lobsters/Swordfish
Food is the #1 item for all pkers, its the most important thing for survival against other players, usually no matter how strong you are.
Strength Potions
Many warriors will use strength potions when pking to deal extra damage per hit, the higher their strength levels, the more it will increase the strength level by. For example, a strength potion will bring level 1 to 2 and level 99 to 118. Higher leveled players usually have more money and will most likely buy more prayer potions than others.
Death/Chaos Runes
Almost every mage uses these runes, for they are almost mandatory for killing in the wildy with magic only.
Wizard Robes, Wizard Hats, Lobster Pots, Harpoons, Rune Scimitar, Rune 2H
Although not too likely, these can also possibly grown in value. Nobody wants to go kill wizards just for their robes or hats, and some people will just buy lobster pots and harpoons off the grand exchange.
Good luck on making your money!
