I'm going to bother someone I want to buy clothes off of
Why would you want to bother them?
I wanna make sure I get the best deal.
I understand, but it's not nice to bother others. You should be more respectful.
You don't understand.  I need a better deal.
If your financial situation demands that you find the best deal, then it is okay to try to bargain with people who are selling things that you need.  However, you should be careful not to go too far.  Make sure you're not pestering this person if they have already said No.