I was wondering if, when you sell a non-quest item to a merchant, can it be bought back even after he's restocked?

Nope, when a merchant restocks, it replaces their inventory.

This. Although I swear I've sold items to merchants before and they stick around for a good long while.

I wouldn't risk it though.

Almost all items get replaced. The few exceptions I've seen are whiterun guard armor pieces in the whiterun blacksmiths' inventory.

I wouldn't risk it either. I think the norm for them to restock is 48 ingame hours.

Not even when you've just sold it to him it seemed to me !

I think this has to do with your presence being with in the area for a certain amount of time as well. After being gone so long away from the area, things change. I noticed a certain weapon not disappearing on the ground until I was gone for probably about 3 days.

Funny thing was, a person asked me if I was going to keep it, and I said no. He said he'd take it and now it says I would be stealing it if I picked it back up. But he never bothered to take it and left it there like I did http://www.gamesas.com/index.php?s=1bff4d1a5195d5570d87e999d29beaba&&app=forums&module=extras§ion=legends#