Essence of Rubeweed isn't potion, but a tincture - hence, it requires Tincturing. And there are probably people selling that tincture, if you want to try it out.
And now we know what the Rube's home street looks like. It would be fun to be able to go there and try to get a trade with him (once per day, naturally).
Yeah, he's a little bit confusing at first. I've hit "no thanks" a couple of times, which dismisses him without going to the next offer.
Note that the Rube is the only way to create dolls that I know of, but they are very rare. So, the two logical strategies are don't accept anything but a doll and accept the first good trade he offers (something valuable for something low-value).
It won't be your last chance to encounter him. (You can get him any time with a Rubeweed tincture (made with a Ticturing Kit from harvested Rubeweed and distilled Hooch.)
Note that you must use it in Ur (the world, not in a housing street).
I was bored one day, so I used a rubeweed tincture per game day consecutively. In the end Rube refused to trade with me... He said something about his mistress waiting at home and disappeared on me a couple of times.