The Sell Center is the building where you sell your hard earned minerals for money. See materials for the full list of prices.
The Manager will allow to you lock some resources so they aren't sold (allowing you to save them for upgrades). After you unlock the Manager, clicking on a resource on any blueprint will allow you to lock that amount of resources in one click. Remember to manually unlock (or reduce lock amount) later!
You can also sell isotopes. But don't sell isotopes!. They are needed for blueprints, Gem Forge and in late game, for the reactor. Lock them all as soon as you get the Manager, and forget about that tab, unless the very rare case that you have serious capacity problems.
Sell Center can be reached through shortcut "S". When you reach the Moon (and on Titan too) there'll be more Sell Centers there. "S" will then alternate between them.