Instant sell is the fastest way to turn a skin into money. A marketplace or cash-out site quotes a price for your item, and if you accept, the site takes the item and credits you right away. There is no waiting for a buyer because the site is the buyer. It then resells the skin later at a higher price.
The cost of that speed is the discount. Instant sell offers typically sit well below the price you could get by listing the item yourself. The gap covers the site's fee, its risk of the price dropping, and the time it takes to resell. Rare or hard-to-price items get bigger discounts because the site is less sure it can move them.
When does instant sell make sense?
It fits when you need money now, when the item is cheap and the discount is only a few cents, or when the market for that item is thin and a listing could sit for weeks. It also avoids the hassle of P2P sales where you have to respond to trade offers.
It does not fit for high-value skins, rare patterns or low floats. Those items sell for a premium to the right buyer, and instant sell prices are usually based on the generic version of the skin. You would be giving away the extra value. List them instead and wait.
How do I compare an instant sell offer?
Check the current lowest listing price and recent sales on the Steam Community Market and on a couple of marketplaces, then subtract the fee you would pay to list. If the instant offer is close to that net number, take it. If it is far below, list the item. The fee calculator shows the net you would keep from a listing at a given price so the comparison is fair.
With CS2 trade protection, an instant sale still moves the item by trade, so the site's bot receives an item that is Trade Protected for 7 days. Some sites pay out immediately anyway and some hold the money until the window closes. Check each site's payout terms at /marketplaces/fees/ rather than assuming.