I would check any forums that have a FSOT spot. I sold my primer set earlier this year over on diaperswappers. And while it does seem like a lot to begin with, you do reuse things. I only sold the primer because my first two are through it and the third one won't need it for a good 4-5 years yet.
Somebody just posted some MathUSee stuff at Waldorf Curriculum Supplies Yahoo group. While this group focuses on buying and selling Waldorf-friendly items, other things pop up from time to time. You can subscribe by writing to [email protected].
I've had the best luck by posting a WTB (want to buy) at this site. I bought the fraction tiles really cheap that way. You get competing offers to sell, instead of having to be the first to get a good deal. Also, I bought used through my local homeschool group got a good deal (no shipping.)
Remember, you can sell it for almost new prices when you are done, so only count the cost of student manuals for long term cost. It's an excellent program that I use for my own dd and for tutoring. Worth the cost!
Be careful b/c there is also a fake MUS Swap group on yahoo. There's no activity on it, so it should be obvious.
Traditional foods eating, homeschooling mom of 2 girls--Miraculously freed of Lyme disease last fall!