If you let people deposit money by bank transfer then fraudsters will constantly do things like get gullible people to send a deposit to buy a second-hand car to the exchange bank account, turn the money into cryptocurrency and move it before the banks get a request to return the money.
A typical business bank account will soon get closed by the bank.
There are many stories from the early days. In the early 2010's there was a money transfer service called Dwolla. It was run by some young men who did not like how banks work and thought they could do better.
Dwolla promised "no chargebacks". They were used by a long-dead exchange called Tradehill. It ended in bankrupcy, lawsuits and fines.
Making a profit of a couple of percent on each transaction is easily turned into an overall loss when banks yoink money back out of your bank account.
The early exchanges had backend code written by wildly impetuous people and really basic security problems like SQL injection.
Leaking all your customers account details through one bug in back in code could get you a big fine.
People selling crypto for cash find that fraudsters will turn up in person with large amounts of high-quality counterfeit bills.
There is the risk of being kidnapped because you can be beaten until you type on your laptop and transfer cryptocurrency.
One screwup can sink an exchange. There was the Bitstamp hack in 2015.
Bitstamp said "we are super secure, only a few laptops can access the back end".
They had Skype and Microsoft office on the laptops! One of the staff opened a Microsoft Office document with macros from a phisher on Skype. That is literally running malicious software. With invisible control of the laptop, the criminals rapidly stole a fortune.
Consider spending three weeks reading the bitcointalk forums right from the start in 2009 to see what a wild-west you are considering getting in to.