In the event of a market downturn like 2008, could I liquidate them whenever I wanted to at good price on a retail stock broker?
Are U.S government bonds completely liquid?
A liquid asset is anything that can be sold quickly. US Government Bonds are very liquid.
In the event of a market downturn like 2008, could I liquidate them whenever I wanted to
at good price
Probably not Maybe. After all, price is a function of supply
During the 2008 meltdown, "everyone" was selling stocks, so there was a large supply, and low demand. Thus, prices fell.
The bond market was affected because some traders needed cash quickly to satisfy loans, and so had to sell their bonds to raise that cash. That increased supply meant... lower prices.
Thus, the answer to "at good price?" is
not really maybe.
on a retail stock broker?
Yes, as long as they are the custodians.