Start with both sides of the trade
Instant buy is the latest reported price paid by a buyer. Instant sell is the latest reported price received by a seller. They describe completed trades; they are not a live list of offers waiting on the Grand Exchange.
The difference between them is the spread. A wide spread is a reason to investigate activity and costs before committing GP. It does not mean a matching buy and sell will both fill.
Check when each price was reported
The buy and sell sides have separate timestamps. One side may have traded recently while the other has not. A fresh page refresh does not make an old transaction new.
Open an item in RuneMarket to see both last-trade readings. Use the price chart to inspect a particular observation, including its reported volume, rather than judging an item from one number.
Read volume alongside the price
Five-minute and hourly windows combine reported trades into averages and quantities. Volume is observed activity, not stock currently available to buy. Averages can differ from the most recent individual price.
Use Compare items to inspect alternatives over the same period. Use the Market map to explore daily movement and recent activity together.
Before you place an offer
Check the item's buy limit, the age of both quotes and how much it has traded. For a potential flip, account for the GE sale tax and allow for price movement. For supplies, choose the quantity you actually need rather than treating reported volume as a purchase promise.