Impact to taxes for retired couple on social security

My husband and I are retired and living on social security and a pension.

 

We enjoy buying antiques and the ones that we don't keep in our personal collection, we sell on eBay. So we will be getting a 1099 k this year.

 

Will we need to pay self-employment taxes? If our standard deduction and the taxes I pay on my pension exceed the amount that would be due for self-employment tax, would we incur some kind of a penalty for not reporting the sales we have on eBay quarterly?

 

Is self-employment tax to help fund social security and Medicare? If it is, since we already collecting social security and my husband is on Medicare, would we still have to pay this tax or is it for entirely different purposes?

 

Also, if we do have to pay the self-employment tax, are the fees that are charged by eBay to use the site for selling deductible as a business expense?

 

Thank you for any clarity you can provide on these topics.