It happens every so often in the world of eBay. The online auction company rolls out a new set of rules, and either buyers or sellers begin raising their voices in protest.

This time, it's the sellers' turn to gripe, as eBay has instituted some new rules to make the site more attractive to shoppers.

Recently, the site has tinkered with its fee structure, search results and feedback system. Sellers say the changes have tipped the scales in favor of buyers so much that it's hurting both sales and the prices sellers get for them.

The reason for the changes: eBay has found its number of active users has started to stagnate.