How about a mechanism that tracks how long ago the item (Unique Items Only) had been sold and then rolls from a probability table with results like:
<<In Stock>>
Yea, the Deathclaw is still in stock, you want to buy it for [blank] gp?
<<Random Spawn Merc>>
Yea, I sold it off to some guy who wanted to kill some undead.
Yea, I sold it off to some guy named [blank].
<<Random City Equipment Store>>
Yea, I sold to a passing caravan.
Yea, I sold to a passing caravan. I don't recall the name, but it had a [blank] emblem on it.
Yea, I sold to a passing caravan going to [blank].
<<Bandits>>
Damn, you just gotta remind me of that do you? I got robbed by some bandits a week ago and they stolen that item.
<<Re-roll>>
I don't have it anymore but if you give me [blank] gp, and I'll tell you what I know.
Piss off, I don't give free information here. Pay up. (CHA too low)//I don't usually do this but you seem like a nice guy, I'll tell you what I did with it.
Then you can gather information in the inn about the whereabouts of the merc, where the caravan is going to, the location of the bandit's hideout.
You can probably either kill the merc or pay a premium to get the item back. Or you are too late, and he sold it already. Or you might find the dead body of the merc with clues as to whether it was a... roll investigation probability table:
<<Boss Death>> (Kill the dungeon boss to reclaim the item.)
<<Bandits...>> (Find the bandits who killed this merc.)
<<Duel>> (Find the merc who killed this merc.)
If you go to the city the caravan was going to, you get to re-roll from the probability table with an added:
<<Caravan Hi-jinks>> (Caravan is missing find its whereabouts.)
After finding the caravan you roll from a caravan mishaps probability table:
<<Bandits??>> (Kill the bandits, rescue the caravan.)
<<Delayed>> (Caravan was delayed, buy the item at the caravan)
<<Dysentery>> (Driver died of dysentery. Relay the message and escort a new driver.)
If you get to the bandit's hideout, they could be alive or dead. If alive, you kill them and search their chest. If dead, roll from your sleuth probability table!
<<Bandits!?!?>> Gang war with their rivals left them decimated.
<<Town Quest>> Some merc finished a Town Hall quest here. Find out who it was at the nearby Town Hall.
So since most of the options loop around, if you aren't lucky (odds for a quick finish gets worse the longer it's been sold), you'll have to go for a rather long item hunt.