For inherited items, your "cost" (actually, your adjusted cost basis) is the fair market value on the date the previous owner died, not the price they originally paid. That means that you are almost certainly selling items at or below their FMV, which means that none of the proceeds are taxable income to you.
You should be keeping accurate records of the sales. At a minimum, you need
- a description of the item in sufficient detail to determine the FMV
- the date you acquired it (the date your grandmother died)
- the FMV on the date your grandmother died
- the selling price and selling date
- any selling expenses (shipping, listing fees, etc.)
Since you do not have a profit motive (even though you have made some cash, this is not a business and you don't have a profit when you are selling for at or below your cost basis) this is not a business and you don't report on schedule C. You can't deduct losses or expenses, but the proceeds aren't taxable, as long as all the items are sold for less than or equal to their value on the day your grandmother died.
If you don't get tax forms (like a 1099-K from Venmo or something) then you simply leave the activity off your tax return. If you do get a 1099-K from a payment processor, you will have to report that as income but then you can report an offsetting adjustment to remove the income and show that it was all personal sales for less than your cost basis. Either way, Keep your records for at least 3 years in case of audit.
