The IRS explicitly states that you need to declare bribes and income from illegal activities [1]
"Illegal activities. Income from illegal activities, such as money from dealing illegal drugs, must be included in your income on Form 1040, line 21, or on Schedule C or Schedule C-EZ (Form 1040) if from your self-employment activity."
Also from stolen goods, unless you return them in the same year:
"If you steal property, you must report its fair market value in your income in the year you steal it unless in the same year, you return it to its rightful owner."
You can deduct business expenses for illegal activities, except for specific categories (for example, bribing public officials is not a valid deduction).
So you might actually be able to deduct that, if you could justify returning the property as a business expense.
Of course not. This is just a clarification about another point of law that a typical drug dealer violates, which gives law enforcement yet another angle.
In some cases proving tax evasion is easier than proving other crimes. This is what got Al Capone -- because the crime was perpetrated through his organization and not by him directly, it would have been difficult to hold him directly responsible for the crimes. However, once his ledger was discovered it was straightforward to prove that he received significant income without making the required income tax payments.
You have to buy them and place them on your drugs so if you get raided, you can prove that you paid taxes on your supply. There are even folks who collect the stamps.
It depends on your definition of "drug dealer". Companies that operate in the open under state laws that legalized weed normally do report their income and pay federal taxes on it, despite the activity being illegal on federal level.
"Illegal activities. Income from illegal activities, such as money from dealing illegal drugs, must be included in your income on Form 1040, line 21, or on Schedule C or Schedule C-EZ (Form 1040) if from your self-employment activity."
[1] https://www.irs.gov/publications/p17/ch12.html