Here's is another persepctive to this..
"retail" usually entails use of a credit card and credit card transactions aren't free. In other words, the merchant (seller) almost always has to pay a fee to the credit card company or 3rd party cc processor. Credit card processing fees vary and thus resellers seem to charge "what they like" as you state above. But it's almost always a factor of their cost + the processing fee they get charged.
Also, it follows that the lower the profit margin (after the cc processing fee is taken out), the higher the setup or finder's (up front) fee.
I hope this helps a little.
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