Just for clarification , is there a deadline, or is it a "once you finish it" type thing.
Also might be helpful to give the word count right off the bat, so people know what they're in for.
Lastly, seems like the offer in return is just a beta reader trade correct? Is there a limit to how much you need to read, and how will you honor it and make sure they honor it? Words can be cheap sometimes. Personally, I don't doubt you, (I have a policy of trust unless bitten) but it might help people feel more at ease if you give something they can rely on. Doing their beta review first is a good way, as you said above, but what if their work isn't finished yet?