This has happened to me a few times. I see the Rube. Click on The Rube and get the "Talk To" option.
Rube makes an offer and disappears. The offer remains. Even if you say "Sounds Great" and accept the offer he is long gone and you receive nothing and the negotiation box disappears. And if your first response is "What else do you have" he also disappears.
Screenshot after Rube vanishes but offer still "available": bit.ly/pxZu1c
It looks like there is a small window of time after he has traded with someone else that the "Talk To" option seems viable but really isn't.