you can improve your rube trades a great deal by dropping any bags of food you are carrying. I have not experimented further, but dropping bags of low level items before you talk to him seems to help a lot. If you don't move, they are locked to you and can be easily picked up again.
You can get:
your value is too rich for me
from the rube if you don't carry any low end items. I've had the 3 dolls but it took me over 3 months of calling the rube every single chance I got when playing, each game day. I must have had over 1000 rube trades to get there. IT CAN BE DONE!
He traded me for high end tools and I think a diamond.
Dolls have two verbs: Pull string, which causes the doll to "speak" one of several phrases (really just appears as a chat bubble if the doll is outside your inventory, or in local if it's inside your inventory), and Pick up/Drop.
(While not mentioned in the OP, Glitchmas Yeti works the same as the other three dolls)
Funny tidbit about the limited uses thing. One of the yeti's phrases is "REQUIRES BATTERIES" and when I first got it, I did start to get worried that it does break at some point (it doesn't).