Click on the Configurations: Global Fees and then click on the green 'Add' button in the upper right.

If you select either 'Premium' or 'Commission', there is an optional checkbox called, 'Sliding Scale Applicable'. There will then be a '+Add Sliding Scale' which can be pressed multiple times to add new rows.

UptoValue = the up to $ amount of this row's increment
Flat Rate or Percentage Rate

Sliding Scale Commission Example:
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UptoValue = $5, Percentage Rate = 30%
UptoValue = $25, Percentage Rate = 15%
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In this example, if an item sold for $4, it would be charged 30% commission. If an item sold for $12, it would be charged the 15% commission.

Sliding Scale Buyers Premium Example:

UptoValue = $5, Percentage Rate = 30%
UptoValue = $25, Percentage Rate = 15%
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In this example, if an item sold for $4, it would be charged 30% buyers premium. If an item sold for $12, it would be charged the 15% buyers premium.