Grand exchange

Every time the grand exchange is updated, if an item is high in demand and low on supply (meaning that there are more people buying that people selling), the price will go up (because it's rarity is higher), if it's low on demand and high on supply then the price will decrease (the rarity dropped). After the new PvP worlds Jagex are releasing come out, people are definitely going to start pking again; these are items that will very like increase in value after the updates.
Manta Rays/Shark/Monk fish/Lobsters/Swordfish/Salmon/Trout
Food is the most common item for all pkers, its the most important thing for survival against other players. Usually no matter how strong you are, if you don't have food than you will have a definite loss against someone of the same level with food. Food is required to fight, to defend, and in order to run away. Even though fishers will well be aware that the food prices will rise and they will start fishing again, more people will go buy the food.
Salmon and trout are usually used by players of lower levels that do not need better food because it is not worth the cost. Even some mid-level players may use salmon or trout when one-iteming to lose less when dying.
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 strength potions than others. A lot of players with prayer will always decide to bring a couple of prayer potions because prayer runs out quite quickly without prayer bonuses. Even with prayer bonuses, it will still eventually run out after a while.