You can, its called a cash account. IMO this is better than a margin account, especially for <20k$. Below this threshold margin accounts are usually treated nearly identical to cash accounts.. except you can _buy_ when you shouldn't be able to... and then have to hold that security for 1-3days for the funds to clear.

In a cash account when you sell, the funds must settle for 2-3days... meaning you can't use those funds for purchases.

It keeps people who are cash short out of the market, rather than in... that's the main difference for small accounts.