is it possible to take out/trade blueprints you have in there already? i have a blueprint i got from an island, and i put it in the craft & construction tab to see if i have the resources for it. i dont, and probably wont for a while xD

but is it possible to take it out to trade/sell, or is it stuck in there forever? 

I think it's there forever.

I am curious to know if Craft items are multiple use, ike the gear and pendant, or if you have to get a new one if you want to make that item again.

I wonder what all the decorations and fancy jewelry are for, if we can give it to the pets or what.

I'm going to follow this.. I want to know as well if we can make multiple craft items

Yes, you can craft multiple items. I made a telescope and the recipe is still there to make more. I just want it out of my inventory now :/

Blueprints doesn't stay in your inventory when you move them to your recipe library. If you see the blueprints that you already have in your library, then it's a spare copy which you probably found while exploring. You can trade it or discard to free the space in your inventory.