If the glider is still in production contact the manufacturer to see if the eBay seller is authorized to sell the plans.
I bought a set a few years ago that ended up being Meister plans just copied. The seller stated they had made the plans themself and did NOT say they were Meister plans in their ad. I contacted Meister and they told me this person was not authorized to sell them and they sought legal action and the guy was eventually shut down and his eBay account closed. Meister even lists several that they have identified as counterfeit so be careful!
Support the companies and individuals who did the research and work to make the plans. Don't support the thieves that steal them, copy them, and resell to make a buck off someone else's sweat.