Your performance is based on the buyers’ feedback on your page.
Buyers can view your seller's feedback score on your listings beneath your username. It’s displayed as a percentage with 100% being the highest score.
So if you have a score of 99.5%, it means that 99.5% of the buyers who left their feedback had a positive experience dealing with you.
In addition to the percentage feedback score, you'll also see a seller's star rating – the number in brackets next to your username. This shows how many buyers have left a feedback for you.