"If I want to sell a secondary treasury, quickly, do I run the risk of losing money? Or is that something for my next experiment with a small amount of cash?"

Eliz,

Any time you buy and then sell anything other than a MM fund, you run the risk of losing money (and even they are not risk-free). So there's nothing exceptional about Treasuries, short-term or not. If you exit a bond position you bought earlier, and interest-rates have moved higher in the meanwhile, you will lose money. Period. Additionally, you'll suffer the spread unless you're selling in size (i.e., typically, a round lot or more). Therefore, bill, notes, and bonds should be considered as "buy 'em and hold 'em" instruments, not as trading vehicles, with this exception. Because bonds can trade at a prem to par before expiration, sometimes it makes sense to capture that prem rather than wait for maturity. However, that's a game that's beyond your current expertise.

As buying in the secondary market as opposed to the primary market, markups aren't what matters. What matters is YTM/YTC/YTW. A shrewd FI investor buys where the achieved yields are highest, never mind having to pay some vig.

Charlie