Does anybody know of (or own) an advertising service that offers a free trial or otherwise allows me to pay by the sale only? I will accept any advertising services that meet either of those criteria. Thanks in advance.
That would be advertisting that is CPA (cost per action or aquisition), many affiliate programs work that way. If you offer your product through an affiliate program you can pay based on sales.
Start by offering a base service, if only simple image hosting or something like that, then run your affiliate marketting system, in order to earn both yourself and your members money. This way, your visitors will help your website grow.
If you use a 3rd party to manage your affiliate program like Commission Junction, RegNow etc... you will have your products exposed to affiliates in their network. In order to get there attention, the product will need to convert well and they will need to earn a reasonable amount of money.
I don't think so. If you want pay by sales,you need to get a bunch of marketer help you market your product. Or you can hire 3rd party to setup affiliate for your product.