There is no restriction on buying and selling U.S. stocks on the same day. (See Figure A below)
However, there is a restriction for Cash Account holders
that they can only use the U.S. sales proceeds to buy U.S. stocks on T+3 (i.e. three days after the trade day). This is to comply with the industry standards regarding "Day Trading Rule" in cash accounts for U.S. securities trading.
Nevertheless, you can still use the "unsettled" U.S. sale proceeds to buy stocks trading in other markets. (See Figure B below)
If you want to do "day-trading" on U.S. stocks, you may consider changing your Cash Account to Margin Account. More
All clients can buy stocks and then sell them on the same day (except for Taiwan, Shanghai and Shenzhen markets). More
Cash Account holders cannot
buy U.S. stocks by using the "unsettled" U.S. sales proceeds.