Trade profit = quantity × (sell price − buy price − tax per item)
Subtract tax from the sale
The standard Grand Exchange sale tax is 2%, rounded down to whole coins, with a cap of 5 million GP per item sold. Some items are exempt. The buyer is not charged this sale tax.
A positive spread before tax can therefore become a loss after tax. RuneMarket's item calculator includes tax when showing estimated profit; use your intended buy price, sell price and quantity to check the trade.
Leave room for price movement
A price buffer reduces the quoted margin to allow for movement between research and execution. One basis point is 0.01%. A buffer stated per side applies to both the buy and sell assumptions.
A buffer is a planning choice rather than a guarantee. On the Flipper page, compare the price targets and estimated return with recent activity instead of choosing an item solely because its headline profit is large.
Size the trade around more than GP
The price gap matters alongside quote freshness, buy and sell activity, the GE buy limit and the GP tied up in the purchase. A thinly traded item may take longer to move or trade at a different price.
Inspect an item chart and its recent trading windows. Compare alternatives if one side of the market is stale. Estimates describe possible trades, while a trading journal records what actually happened.
Keep estimates separate from results
After buying, record the quantity and actual purchase price. When selling, record the actual proceeds so your journal can calculate the realised result. An unfinished position is still GP at risk; it is not completed profit.