Retaliation means to take revenge.
"In retaliation, a user leaves a negative reputation score to another user, because he received a negative score from the latter. This opens the way for extortion, and in general, it discourages honest users from leaving honest negative scores. A variant of retaliation is engaging with a high-reputation seller and misbehaving, then blackmailing him with the threat of leaving him negative feedback if he does not leave positive feedback as buyer. This activity is called 'feedback extortion' on eBay's help pages."