Wait! ....I've revised my vision of this!
I'm thinking of the FreeCycling model now. Instead of "first in line" gets it, it's a more fair system, and I think a better one all around.
First of all, if the yarn is very valuable to you, you might want to sell it or something, outside of this system. This is for yarn you don't have an attachment to.
You post: "OFFERED: 3 skeins of Homespun brown". Then, everyone who's interested emails you. Emails can vary in terms of enthusiasm or need, and that often will influence who you want to give it to. You choose who you want to give it to, and when the deal's done, you simply post: "TAKEN: 3 skeins of Homespun brown." People all assume the yarn went to the best home possible; and in any case, it's fair that the person giving it up would decide.
I like it because the system is designed to please the giver of the yarn, which encourages giving. And it also increases the chances dramatically that the yarn is received by someone who'll really love and use it. With the box system, you only have a limited selection to choose from, and you may not be that excited about what you take out. I think it can end up just moving the yarn around, without necessarily finding enthusiastic homes for the yarn.
Is it fair in terms of getting back as much as you give? Well, if you really want yarn, you'll be sending out more emails to get yarn. And if you're frustrated because you haven't gotten anything after giving, you can express that to increase your chances of getting. What's cool, though, is that some people will prefer to give stuff, and won't necessarily want yarn offered, and they'll be fine with having given more. And some are more in need of receiving. I like that the system allows for this, and tends to keep everyone happy.