I'm wondering, if you're on a brokerage site that offers "real-time" ask and bid quotes, are those ask and bid quotes you see flashed on the screen necessarily the best ask and the best bid available, or are brokerages limited in their ability to find the best ask and best bid at any given moment? Are some brokerages better than others in this regard? Or is it a matter of paying an extra fee to see the best ask and bid? Thanks
If you are looking at a security trading on a United States exchange, then the SEC's rules for National Best Bid and Offer apply.
Brokers are required to execute customer trades at the best available ask price when buying securities, and the best available bid price when selling securities.