1099-K income from eBay, estimated taxes?

It's called "estimated" taxes. Each quarter you determine what you think an appropriate payment would be. The object is to pay enough so that you don't owe more than $1,000 on April 15. The last quarterly payment is due January 15 of the following year. So if you have been keeping track, you can make up any shortfall at that time.

At the end of each quarter, figure out your annualized income based on how much you've made so far and pay estimates based on that.

If your income goes up later in the year, use form 2210 when you file your taxes to explain the uneven estimated payments.

I agree with Stu. You can go to irs.gov and either:

1. there is a button on the main page, to the effect of pay my taxes where you tell IRS you want to pay estimated taxes for (example) Form 1040, which year, name, address, SSN, $$, how to pay (debit bank account, credit card for examples), or
2. go to Forms and Publications, select Estimated Taxes, Form 1040-ES, select year, fill in similar info, print, address envelop, insert form and check, stamp and mail.

Understand that there may be a timeliness element, in some circumstances such as an audit. In general, income, net of allowable deductions and exemptions, estimate income and self-employment tax by calendar quarter, could produce a calculation of your estimated tax due:

1. January, February, and March by April 15 (this year Ap. 18);
2. April, May, and June by June 15 (Not July 15);
3. July, August, Sept by Sept. 15;
4. As Stu stated, Oct., Nov, Dec., by Jan. 15.

Are we having fun yet?

And if your income is low in the first part of the year and high in the second part of the year, so that your first payment turns out to be quite a bit below 1/4 of the total tax, you can file form 2210 next April with your 2023 tax return and probably avoid a penalty.

Finally, if you also have a regular job, you can avoid paying estimated taxes by simply having your withholding increased. A nice feature of doing it that way is that, unlike estimated tax payments, all that matters is the total withheld during the year, not the timing of the withholding.

Good comments from all the others.

Will the safe harbor based on last year's tax liability help in both the first and/or subsequent years of this business?

A safe harbor is just that - it can be used for any set of circumstances.

Ira Smilovitz, EA Leonia, NJ