Is it possible to repricer only with specific sellers in the repricer?

Yes, you can set this when configuring your repricer rules. You can specify whether you want to exclude specific sellers in your repricer rules, or whether you want to compete with specific sellers. EffectConnect will then only look at these competitors. 

To read more about this, view the repricer document via this link. Scroll down to 'Good to know about excluding sellers, or just competing only with specific sellers'.