The marking pellets in the picture were oil based so the paint stayed on the cow / tree you were marking, they were used at first for shooting at each other but then the formula changed so they would be less permenant. This kind of pistol is generally loaded from plastic ten shot tubes like the ones you were looking at on ebay containing 10 balls (rather than the pellet shaped "first strike" ones in the ebay images you linked).
There are collectors about, more in the states than here but this particular model tends not to attract a massive premium since there was a lot of them sold. That's the first of that particular model of pump handle I've seen though which could add value.
If you are interested in selling I would suggest asking on
www.mcarterbrown.com , a US and Canada based forum with a particular interest in older markers, what they think its worth and what they would be willing to pay, then try advertising it at the same price on the for sale section on here (once you have the requiste 20 posts) maybe taking into account the difference in postage that you'd have to shell out for getting it to the states.
