Hello i'm developing this website https://buysellcarwithbitcoin.com/ how can i display the USD price of classified ads also converted in Bitcoin ? thanks!
If you want that cars could be paid by Bitcoin you need some payment gateway.
Otherwise, if you want just to display prices in Bitcoin you can use some blockchain.info API to get the price for it. For example, this URL will return you how much is 5000 BTC: https://blockchain.info/tobtc?currency=USD&value=5000
Some bitcoin exchanges will offer an API to return the current BTC price, so you can pull data from them, divide the item price into btc price, and display the resulting btc price to your customers.
A quick search brings up a Coinbase API for btc price that would probably work for you.