Sell it where? Ebay?
The VeRO programme on Ebay will prevent you doing this, in the same way that FIFA Ultimate team players aren't your property to sell, nor is any content that is owned (regardless of being modified/added to by you our someone else) by SI.
There was someone selling custom transfer updates for previous versions of FM, SI stopped this i believe.
SI believes in a free and open community, they really don't take kindly to anyone making money off the back of their content. Donations to keep fansites like our own are necessary parts of todays tech hungry world, and SI are fine with this. But making money from something like graphics, transfer updates or guides/walkthroughs (See FM-Britian and the fallout from their communication and psychological warfare guide that they charged for - FM-Britain are no longer an SI affiliate) isn't something they want to happen.
The CM/FM community was founded on free and open custom content/help for everyone, that should never change.