To add to your own answer Robin, you can also simply flatten all the layers in Illustrator and "divide" the file using the pathfinder tool. That is, if you really need to share that artwork as a vector.
This is what a LOT of artists on stock pictures sites do to their vectors and only people who have a lot of free time will try to "re-assemble" your vectors. Already by doing this you make it more complicated from them and the quality of printing will not be affected. But keep in mind any vector file has the benefit of being highly editable so if you really want to prevent anyone from moving things around, your PNG option was good; just make sure it's 300ppi resolution if you plan to sell it for printing.
Watermark and PDF security will be absolutely useless for you if you are actually selling these graphics! You'll simply make your
customers very mad.
You can also export as PDF and create a flatten high resolution (raster) out of your artwork; this is a good option if you have a lot of images to convert and like to use the PDF format. See this link for how-to: How can I protect my job from a client publishing it elsewhere?
OR you can also simply "rasterize" your artwork directly within Illustrator (menu Object > Rasterize...) at high resolution and save it as eps. Even faster, you can use the "export" and save it as a tif/png. But just so you know, when people ask for a eps file, that's because they want a vector... so might as well simply save as tif or png if you plan to provide a raster image.
If the group you are showing your images to isn't actually purchasing them, simply show them a JPG at low resolution. There's no reason why they'd want an .eps file unless they actually want to grab some vectors in your file!
Also, it's a common practice to sell a raster high resolution (eg. a tif, png or a jpg at 300ppi) for a normal price and the vector file (eg. a pdf, ai, svg or eps) at higher price. Some vectors are sold 2x the price of a normal flat raster image.
It's not a bad idea to also include your own terms and a license if you are selling your artworks/posters/vectors yourself. That's not a magic protection but it works the same way as a contract.