After most missions you are bound to have obtained a limited amount of SWAG. This is generic loot which can be stolen from your victims.
Normally you can sell good for 30% of their price.
Anything which you looted or stole from someone suffers a 20% penalty. This reduces the sale price to 10%
Armor or clothing is penalized for being used. This penalty makes it unsellable. There may be specific circumstances in which this is not the case, but unless explicitely told otherwise assume you CANNOT sell armor.
Regular ammo is also unsellable. This is because of the combined penalties for being stolen and an additional 10% penalty for the market being flooded. There is no shortage of ammo. Exotic ammunitions on the other hand might be worthwhile.
For simplicity it takes 1 day (no rolling) to sell all items you obtained which are valued 100 nuyen.
When the mission summery is posted after a mission, it will include the loot available to everyone and the resell prices (as well as any time requirements).
The key thing to consider about loot is that its not designed to substancially supliment your income, its designed to provide you with easy access for goods which might be helpful.