I have to ask clarification because the OP seems to be contradictory. "Cash" implies physical bills. "Over the internet" implies the buyer will never meet the seller in-person.

If you mean to use a venmo-like fiat payment system to pay, then robosats is what you're looking for.

If you mean to find people over the internet that you can meet F2F & IRL to exchange bitcoin for fiat bills, you can do this without any bitcoin exchange service. If this is the case, why not just use your lightning wallet?