So why are you worried about the amateurs who sell one photo in their life? Do they destroy someones livelihood?
Edit: perhaps it's the amount of one-time-sellers that is threatening?
That's a good question indeed.
But why skip the opportunity if you someday have the chance to sell one?
Aah, coincidence... I'm just in that situation now.
At the moment, a publisher wants to buy one of my photographs for a book cover and they asked me for a price.
I asked an amount that I thought the image is worth for me... and they accepted it. I had a bit of work editing the image, photoshop, getting the right dimensions, dpi, ..
I could buy me a (cheap) lens with this money, but it's definitely not enough to live for a month.
I hope the amount wasn't too low to embarras pros who do it for a livelihood.
BUT, the problem as an amateur is that in my country, I cannot just make a fake invoice, as I'm not registered as a company.
I can let make a friend of mine (who does own a company) a legal invoice, and hopefully get the money indirectly by him.
(I surely want to start a company someday, but only if I could live from the income, if I would sell more.)
On the other hand, I don't want to skip this opportunity just for that reason.
I guess lots of pros are rolled into this by once selling an image as a one-time-seller, with a hope to get more images sold later on...