The Thinking mother wrote a great blog post about the resale value of Math-U-See, here is what she said,
I can’t believe the prices that these are selling for, on EBay.
The average selling price for a USED (but complete) “starter set†is $28.25. A brand new set bought over the internet direct from MUS is $30.00. The crazier thing is that one auction was for full price and two auctions of USED sets exceeded the price of new sets. I don’t understand this!
Meanwhile complete sets of used “completer sets†averaged $29.67, while full price new is $35.
Prices for incomplete sets were only slightly lower. I am amazed that people are paying such high prices for used items for which they also must pay shipping. It is not as if the sellers were offering free shipping or some other perk! Why are people not buying directly from Math-U-See?
Read the full post here.
I’ve seen this myself on ebay and other places, I once saw a classic Intermediate teachers set going for over a hundred dollars! 
Well there you have it folks invest in Math-U-See for your kids college fund
just kidding, it is a good investment though for higher grades in college
It is crazy. I had looked for a long time on ebay as well as through other used curriculum sites and in the end decided it was simply better to buy new from MUS. I was planning to buy at the local homeschool fair this spring but after thinking it through realized that it would be cheaper to pay shipping and have it delivered to my door than to pay the sales tax at the fair and have to lug everything around too.
Just goes to show what a great product this really is. Thanks.