I got dozens of Greenstone, Phenalope , Aventurine etc etc but I've found no use of them, is it okay to sell them?

Also, how to get rid of items that got 0 value? I can't sell them because the seller won't accept them.

Sure, that's the idea (they are just like the little rinky-dink gemstones you found in BG1 and BG2). All they do is tie up inventory space (not much, but hey, gold doesn't and doesn't weigh anything).

It's called littering. :D Just drop them all back at the temple; if it turns out you need the item, it will still be sitting right there on the floor. Also, if you lose track of an item you still need, go to the pool in the temple (right near the portal you come in from when you use the stone of recall) and it will show all quest items that have been dropped on the ground by you. For some gold, you can get the item back.

ahh..the pool yes. I was wondering what's the use of it. so, it give back lost quest item then. I see..thanks.

If the sell value on something is 0gp that means that it is a quest item, so it would be a good idea to hang onto it.

Thats tru Mossy, but not tottally, because ive had a lot of arrows and bolts that i couldnt sell (0gp) some various other items too that were pretty common, maybe its just a bug.

Most non-ammunition items that have a sell value of 0 are quest items. However, each henchperson has a quest for each chapter (I assume, only in chapter 2). If you're only doing the quest for one of them, you'll find five excess items. Plus, there may be other quests you aren't eligible to do, because of your class, and you may still stumble onto some of those items. Just find a container near your portal and store things your unsure of there.