Simply put, a tax write-off is any legitimate expense that can be deducted from your taxable income, when you file your tax returns.
Also, according to Wikipedia:
In income tax calculation, a write-off is the itemized deduction of an
item's value from a person's taxable income. Thus, if a person in the
United States has a taxable income of $50,000 per year, a $100
telephone for business use would lower the taxable income to $49,900.
If that person is in a 25% tax bracket, the tax due would be lowered
by $25. Thus the net cost of the telephone is $75 instead of $100.
And as for consigned to specialty sections of record stores, the most probable meaning is that prior to Nirvana and it's immense success, any form of alternate music was not in great demand and, as such, was usually placed only in the specials section of record stores, away from the (then) mainstream music, if you will.