Interesting that you brought up that question. Within the last couple months I finally got around getting my yarn stashed on Ravelry. A couple unexpected things happened.
First, I have received multiple requests buying yarn I have stashed but not listed as sell/trade. I don't really mind but it had never occurred to me that people would do that. I understand the frustration of desperately needing yarn to finish a project and would be glad to help if I can.
BUT, two things have happened that bothered me. In one case I went to a great deal of trouble to have the yarn weighed to determine the cost of postage and researching on line to come up with a fair price (I was asking slightly more than half what the retail was) only to have the person who approached me say they had found the yarn elsewhere. It was time out of my busy schedule for something that wasn't my idea.
The second is that I have received an occasional offer to buy yarn at a price that is significantly less than I paid for it. If I were trying to sell yarn and had listed it for trade or sale then I would be open to "bartering". But I love my stash and value each skein (corney I know) and if someone is desperate for something I have I don't think they should be trying to get it at rock bottom prices.
So I would say, if someone has yarn you truly need don't hesitate to ask but be sincere in your offer and don't expect them to slash prices.