I don't want to withdraw USD, is there any exchange or site which allows me to sell bitcoin and keep USD in their account, so i can buy and sell bitcoins.
I can buy bitcoins from somewhere else and transfer them to their account.
I am using Xapo for this purpose. I am not finding any difficulties with them. Xapo will create a fiat wallet for you when you will be selling your bitcoins with them and you can hold your fiats as long as you are preferring.
This may not be like actual trading but very similar to dealing with a personal exchanger at preev rates. Yes, you will be able to always deal at preev rates unlike any other exchange. I guess this must be a big unique advantage I am simply enjoying due to Xapo services.
(But please remember Xapo needs you to clear different verification levels based on your volume. )