It depends on the brand and the condition. So say for example you have a pair of Levi's jeans for a 2 y/o.. Once they enter the brand, condition and size into their computer which generates the value/price that they can sell it for which may come up to 9$, you will get a 3rd of that which is 3$. That's with anything that you sell to them. Their system generates the price of what they can sell it for and you recieve 3% of that. So If they can sell it for 90$ than you will make 30$. I hope I kinda gave you a good idea of how they operate and pay.
It all depends on the brand and the popularity of the item (s). You only get a percentage of the selling price.
It all depends on what it is, they price it and only buy it at a certain percentage.
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