Is it possible to travel the the USA on an E-2 visa with a one way flight ticket (no return), with the intention of purchasing the return flight at a later date within the allowed duration of stay? For example, within 1 year.
Yes. You may have to convince the immigration officer that you will comply with the conditions of your admission in E-2 status, including having the means to purchase a ticket out of the country. But in any event a round trip ticket is not required when you enter.
As the USCIS explains, your initial entry to the US in E-2 status is for two years, and you can extend it indefinitely. You are required to leave eventually, but you aren't required to have a return ticket at the time you enter.