What's the vig, the points, the gravy that gets the buyer to buy my dirty old used Treasuries instead of new ones?? Has anyone sold and what kind of discount did you run into?
Your sale (if large enough quantity) should be at about the same YTM as what a new bill would yield with the same duration.
For instance, using times to make the math easier. Let's say you bought a one-year T-bill at 2.04% rate (2.05% return) which would have cost $98.00 per $100.
Now assume you are 6-months into that, and decide to sell. Let's assume that rates have jumped to a 4.09% annual return. Well, your T-Bill would be worth $98.00 (Because a six month T-bill w/4.09% YTM would have a discount price of $98 per $100.)
How about a case where you would lose money? Easy - let's say the rates went from 2.05% to 5.14%. Your T-Bill would need to price at $97.50 per $100 to give a 5.14% YTM over that six-month period.)
ETA: A case where you make money but less than if rates stayed even: 6-month YTM @ 3.05, your T-Bill would price at $98.50 per $100.
ETA: A case where interest rates dropped, e.g. 6-Month YTM @ 1.01%, T-Bill price is $99.50 per $100.
You can sort of use the T-Bill rate calculator:
https://goodcalculators.com/treasury-bills-calculator/ to play with the price paid field to calculate the resulting YTM on a six-month remaining bill.
I'm too lazy (and busy grading endless stuff) to create a spreadsheet for this, thus the link to the calculator.
This should give you an idea of a "fair" price bid if you were to sell them on the secondary market.
SIDE NOTE: WARNING, Headache producing.
Also note that calculations for long bonds can be much complicated in terms of OID (original issue discount) and market discount from par. Here's a write up if you want to punish yourself in understanding the complicating factors:
https://s3.amazonaws.com/static.contentres.com/media/documents/990086a6-d87e-46df-bdb4-681554969ee5.pdf
