You are taxed on your profit, not the total income. Selling online is a business. On your tax return you will report your total income and your expenses. The net business income that is taxable is the total income minus the expenses. Expenses include what you paid for items that you bought for resale.
Income from selling personal items for less than you paid for them is not taxable, but you cannot claim the loss. If Mercari sends you a 1099 showing the full amount that you received, you will have to indicate on your tax return how much of it was for personal items sold at a loss. That amount will not be considered business income.
You will have to pay income tax, plus self-employment tax, on your net business income. Even though that will be less than the full $7,000, you should be making quarterly estimated tax payments so that you don't have a big tax bill, plus penalties and interest, when you file your tax return.
Filing separately from your husband will make your tax more, not less.
