I had contacted a website once asking about getting in on their advertising gig, but realized that I couldn't afford to do it as they asked you to pay them (fair enough). This had included a shop credit that you contacted the owner of the site you were advertising on to get paid for your items. I couldn't afford to do it, so I declined. A couple of weeks later, the game had changed. You didn't have to pay anymore, you just provided a store credit. What hadn't been made clear was that you would not be reimbursed for your items. That's right - you provided a store credit and if the person who won the credit spent more than the credit was worth - you PayPal invoiced them for the balance. Otherwise you just send out your items - with no actual payment for them at all!
Now had this advertising method generated any actual sales - I wouldn't have had a problem with it, because I'd have had the money to do what I needed to do. But it didn't. I did not get a single sale from this except for the sales from the credit.
Now the gal who runs the site I was advertising on offered to pay me the money for the store credit I'd provided because it wasn't made totally clear that there was no more reimbursement. And that's cool. Now I know and I'll be prepared next time I decide to advertise there.
But lesson learned - always make sure you know what is going on - especially if the format has changed since you last looked into it!