In general, sellers only need to collect sales tax if they meet the legal definition of "reseller". The test varies from state to state, but is generally a combination of gross annual revenue from sales and number of sales transactions per year. Generally, transactions between private parties are not subject to sales tax.

eBay was hit by special "marketplace facilitator" clauses in remote tax laws that have popped up since South Dakota v. Wayfair. The tax is actually imposed on the marketplace, not the seller. It doesn't matter what the seller's status is relative to the above, because it is eBay that is being required to remit he tax. If the seller is independently obligated to remit tax to a given state as well, they count the eBay-collected tax against their own obligation.