"Instant Buy/ Sell" is a common term used in the crypto community. At it's very basic it refers to the process of immediately fulfilling an 'order' to buy or sell a crypto-coin on an exchange. Which means, you do not have to wait-- you get your crypto (in case of buy) or your funds (in case of sell) in your account almost instantaneously (well- it's actually as long as the computers decide to take).
The way the order is fulfilled defers from one exchange to another.
On Zebb, we do not buy (or sell) Bitcoin/ Ethereum from you. We only 'match' you with other people who want to sell (or buy) Bitcoin.
Before we go into what happens when you place your order on the Zebb platform, let's take a look at how buying and selling happens through an 'order book' on the main Zebpay platform.
A person, let's call her Radhika, wants to buy Bitcoin at a certain price. She logs into her Zebpay account, and creates an order specifying the amount of Bitcoin she wants to buy as well as the price at which she wants to buy it. The order that she has created is listed in the 'order book'. When multiple people create many such orders, all with different quantities and at different prices, it fills up the 'order' book. All of these 'buy' orders are arranged according to decreasing 'price'.
Now someone else, let's call her Maya, wants to sell her Bitcoin. She doesn't care what price she gets for it. Maya logs into her Zebpay account and creates an order specifying how much Bitcoin she wants to sell and then selects the option of 'Market' price. The Zebpay system checks the order book and finds the highest price (at that exact moment) at which someone wants to sell their Bitcoin . For the purpose of this explanation, assume that Radhika's order is the best price that Maya will get for her Bitcoin. The system then matches both their orders and creates a transaction between them.
Radhika has bought Bitcoin and Maya has sold her Bitcoin. It's that easy.
So what happens when you place your order on Zebb?
All of this happens within the span of a few seconds.
There are thousands of customers buying and selling on both the Zebb and Zebpay platforms simultaneously. It is common for the 'Price' to constantly vary by a few rupees, even within the fraction of a second. Therefore, the price that was displayed to you at the beginning may have increased or decreased by a tiny amount when your order was completed.