I'm still confused as to your exact role. It sounds like it's obvious that you've been hired to help setup their EBAY sales operation- but what's to say that that they just don't cut you out of the equation once it's setup?
In the beginning, what arrangement was made to get you paid? Was it a setup fee, plus commission for cars sold? If they have "hired" you as an employee, I'd want to be on a weekly or bi-weekly pay cycle, based on your sales. If you're contracted as an outside entity- the invoice approach would work- but once you've handed over the passwords, you've essentially given them they keys to the operation. After all, they know how to sell, you were only providing the technical services.
Like John said, be wary of car biz guys. I grew up around it and a lot of them play fast and loose with the rules.
"The first lesson of economics is scarcity: there is never enough of anything to fully satisfy all those who want it. The first lesson of politics is to disregard the first lesson of economics." Thomas Sowell