DMarket's own Instant Sell is always lower then selling to active Targets. That gap is usually around 5%. Here's how to get the higher price, just as fast.
From your DMarket inventory or items-on-sale list, click into the item.
On the item's page, click Trading Statistics next to Sales History. This shows the live order book
In the Target Price table, note the top number (the "Max Target"). That's a real buy order already sitting there right now, waiting to be filled - not DMarket's own lower quote.
Go to Sell, set pricing to Customized, and type in that same target price. Because a matching buy order already exists, the sale clears just as fast as Instant Sell would just for the higher amount.
This is general guidance, not financial advice - order books move, so double-check the current target price before listing. Screenshots are illustrative; DMarket's own interface may look slightly different.