Watermarking, but sufficiently bold as to show when photocopied.
This could be placed at even intervals by software, or what might sound better would be to place them at random intervals where it does not cover any detail, but can run through a line. However, if you do avoid details, then people can tip-Ex out the waterline.If waterlines run diagonally they can't be confused for instructions.
Watermark should include the text that this is copyright property. Me personally, I would put in large text (200 point) across the plan "These plans not for sale on Ebay. If you bought these plans on Ebay they are stolen". That way Ebay will have to act.
You can also put, original plans available from ............
Also, numbering the plans, so that if they do turn up it is traceable. It would also be fair to point this out on the plan somewhere "these plans are numbered to avoid IPR theft. Copying of these plans is not permitted, copying is traceable. The plan number could also be part of the watermark. This is a bit more laborious.
At the end of the day, you cannot stop a modeller knocking off a copy for a mate, but you can reduce wholesale copying. And make it very difficult to flog on Ebay.