SellerLogic, and I have never regretted it. Before that, I tested Threecolts, but SellerLogic https://www.sellerlogic.com/en..../versus/sellerlogic- turned out to be much more effective in terms of the speed of reaction to changes in competitors' prices and retention of the Buy Box. The interface is intuitive, everything is clear, and the strategy settings are flexible and thought out in detail. I especially liked the ability to analyze in detail price fluctuations and competitors' behavior - I have not seen such precise tools in any other system. Thanks to SellerLogic, my sales are growing steadily.