It's the major headache how to process this ingame and how to handle non-monetary loot...
Oh that's easy.
You can go to any local IRS branch with the loot/list of items for an appraisal.

What's that? You don't want to walk into a federal building with a prototype or list of the items you lifted from your last corp raid?
Yes, that can be annoying, but honestly so long as you didn't rob a federal facility in the process and pay your tax they really do not care, because just like the historic Minnesota Grass tax, Revenue officials must guarantee confidentiality to those paying.
The right to pay anonymously is important to ensure that individuals are allowed the constitutional guarantee against self-incrimination.
And if taking a list because you don't feel comfortable hauling in the loot,please try to describe the items to the best of your ability to ensure the best possible assessment.
Otherwise if you just put down say, 10 automatics lifted from security guards without listing brands, the assessor can and will assign the highest possible value for that category of weapons even though they know full well that particular corp uses cheap knock-offs.
Also make sure the list is disposable, I recommend flash paper, because while the IRS can't disclose anything, the cops sitting down the street might stop you for a chat and you don't want that list falling into their hands.
Again they can't ask what you were at the IRS for and the IRS can't tell them, so just make sure there is nothing on you can be traced back to anything.
Services rendered to your friendly neighborhood money launderer.
But that is the rub, the fence/money launderer is never going to pay you the full value of the item and your tax guy isn't going to care that YOU only got 30% of the value of the item, the IRS will still want the tax based against the listed value of the item.
It's like those poor bastards who win a car on a game show only to get slapped with a punitive gift tax that is still based on the value of the car and not the actual price you might get for it.