I ran into the same thing. The last "Larceny" item you get will be "bugged". If you'll notice, the next to the last item you picked up will say "Return xxxx to Delvin", and "Return Falmer Eye Completed".
Something like that. The jist is, the one before the last is also activating and completing your last.
Spoilers, so I'll add tags. Spoiler:
If you're at the door where you fight Mercer, go back out into the world map and get the rest of the "Larceny Targets" and sell them to Delvin.
Kick Mercer's butt and loot ONLY the skeleton key. Don't worry Mercer isn't going anywhere for a while...lol. You can come back later and get the rest.
Return to Delvin. Punch up the console "`", then type in: setstage TGLarceny 80
Then talk to Delvin and select the Falmer Eye option. This will complete the last Larceny quest. You can then go back and dive down to get BOTH eyes off Mercer and sell'em or whatever. And one will still show up in the bookcase behind Mercer's desk too.
I have this problem, only it's not with the Falmer eyes, I can't sell the East Empire Company map, because I missed it while I was there the first time, so I have this bug, just on a different item. Any way to fix that?
So you just need to make up for that one item? And you've been able to sell all the rest?
I can't say for sure, but I would try this:
If you've gave the Falmer eye to Delvin already, and have gave him all other items except the map. I would just go up to him, bring up the console and type in:
setstage TGLarceny 40
It should then pop up, "Take the East Empire map to Delvin". Then just talk to Delvin and the option should be there. Just do not have the map in your possession at all. And be sure to save before trying it.
If this doesn't work, the only solution I know of is the one I posted above. Where you need to get all the items turned in first, the skip the Falmer eyes and use the console to activate that quest.
If you've got the Falmer eyes long ago, you may be stuck. Good luck.