No, that's not possible. In TW1 it was, but no more in TW2. Keep the ones you need, sell the ones you don't.

Not sure, but you could always try it. Just save you game first, sell one, try and make the potion or item, and if it won't let you just reload the save from right before you sold it.

The first response is quite correct; you MUST have the formula or diagram at time of item creation. This is unlike books, which have no function other than adding information to your character's profile and for which you have no subsequent use.

There's really little reason to lose the diagrams and formulas, anyway. Their resale value is usually minimal; they weigh absolutely nothing; there are no volume-based limitations on what you can carry; like all non-expendables and even the flimsiest furniture they are completely indestructible so you will never find them destroyed just because something set you on fire (and for this reason you gain no benefits from having multiple copies); and there is only a single formula that a single NPC will want from you in the entire game and it's not obligatory that you actually yield it.

The only reason I sell diagrams that I don't use is to save scrolling through them.

Apart from that, there's no reason to as they're worth almost nothing and weigh nothing.

Thank you very much all of you!