Buying on ebay

When making an offer on eBay how low is it okay to go without coming off as rude? I just purchased a coat on eBay, the seller was asking $100 so I offered $50 and they accepted it. I figured 50% of the asking price was reasonable. But after seeing that they accepted it so quickly I’m wondering if I could have went lower. Buyers how much do you usually offer? And sellers what is the lowest you’d accept?