As far as I understand this should be something like a "buy it now"-option that is availiable on eBay.
I understand this might be helpful sometimes, but how do you define the markup that has to be paid for instant transfers (+who gets this money)?
For example:
Player is listed for 1.000.000 and a buy-him now price of 2.000.000 - the instabuy would be much more expensive (as demanded) and at the same time much more lucrative for the seller, which at the same time would result in a drop in demand for such transactions...
I would only buy such a player if I was sure that the price paid was near the market value, otherwise I would search for another option. And to be honest at the moment there are not many unique player types left, so an alternative is waiting around the corner everytime...