Hi Mike, In the software industry we come across this issue all the time. If you are not particularly bothered that somebody is profitting from your work you might just leave it there.
However, if you would like to protect your work from profiteers, whilst still offering your work for free for non commercial purposes then you might want to consider one of the 'open' licenses.
One I have personally used for software source code that I have shared is the
Creative Commons license. You can tailor it to meet your specific needs and you could use it for plans or other content. You can generate your own license from their website, nice people that they are. :)
Also it is a myth in the software industry at least, that unlicensed content can be freely distributed. Unlicensed content simply means the originator has not given you permission to use, distribute or make profit from it. So content with no license is simply ambiguous, and you may not do with it what you want without the originators permission. I'm sure this will apply to your content also. A restrictive license may be applied retrospectively.
Finally eBay have a duty not to allow protected material to be distributed without permission. Should you decide to contact eBay and find them unhelpful (as is my experience) I am sure your many grateful fans here will be happy to bombard eBay with complaints. Should you wish to 'set the hounds' on them just let us know.
We are very grateful for your hosting of plans and the forum and I am sure many people here will do whatever they can to help.