Yes, I think think that's the reason.
It's even worse than that... Brian's assessment is pretty accurate. These are just a couple of randomly-selected examples:
Example 1: "Aikido for Life"
is stickered at $12.95. Half retail would be $6.48. Amazon marketplace has new copies for as low as $0.01 (!!!), and Ebay has new copies on Buy-It-Now for as little as a dollar.
Example 2: "The Spirit of Aikido"
stickers for $10.00. Half retail is $5.00. Amazon marketplace has new copies for $3.94. Ebay is as low as $3.95 for new.
Used copies are even cheaper in most cases. The sad reality is that mainstream titles usually have very little cash value until a piece achieves some semblance of collector status or simply becomes difficult to find. It's all about supply and demand.