Here are seven items that often are near, at, or below general store buyback prices. RuneScape players can buy them in bulk on the grand exchange and then spam click the sell button to sell to the general store in loads of 500 ea.
Polypore spores: Polypore spores sell to the general store at three coins each. An offer parked at one or two coins each can yield up to 60k every four hours, assuming the offer actually buys. Because the current price is two coins each, an offer parked at that is more likely to fill every time if they don't buy at one coin each. The buy cap is 30,000 every four hours.
Runes buy at 25,000 every four hours. Chaos runes sell to the store at 42 each, death runes 93 each, and soul runes 123 each. When these items are at or below these prices, buy them up. Because they sometimes rise above the store value, they can be held and sold on the grand exchange for a greater profit, but this requires holding them for a greater time and tying up cash.
Hand cannon shots buy at 150 each, rune arrows buy at 153 each, and royal bolts buy at 45 each. If these items can be bought below these levels, the RuneScape player can turn a good profit. Royal bolts seldom hit their floor, and sometimes double in price. Therefore, these are best bought and held, then sold on the grand exchange.
RuneScape players make a small amount of money on a large amount of capital this way, but the money is guaranteed.