I had a quick look and didn't find anything similar so if this is a repeat request then i apologize.
I find that every time i go to sell any loot i have to tiptoe around my other precious item so not to sell them to a merchant by mistake, this happened to me a wile ago when selling a unused sack, i clicked to sell it and nothing happened so i clicked again then it sold the sack and the item below which happened to be my glyph sack with all my lovely glyphs all gone.
I think it would be useful to have some sort of tick box in a items description to prevent it appearing/been sold, in the sell window.