Tax Year Prior to 2020: eBay sales considered a business?

It depends, please see the list below and apply them to your situation.  Essentially, if you intend to continue this to earn extra money (profit motive), the IRS will likely consider it a business.  If you are doing it solely to clear out clutter, it would be a hobby.  This IRS Article has some tips about Hobby or Business?

Generally, an activity qualifies as a business if it is carried on with the reasonable expectation of earning a profit.  In order to make this determination, taxpayers should consider the following factors:

• Does the time and effort put into the activity indicate an intention to make a profit?
• Does the taxpayer depend on income from the activity?
• If there are losses, are they due to circumstances beyond the taxpayer’s control or did they occur in the start-up phase of the business?
• Has the taxpayer changed methods of operation to improve profitability?
• Does the taxpayer or his/her advisors have the knowledge needed to carry on the activity as a successful business?
• Has the taxpayer made a profit in similar activities in the past?
• Does the activity make a profit in some years?
• Can the taxpayer expect to make a profit in the future from the appreciation of assets used in the activity?