$49 is the annual subscription fee for eBay Plus.

Presumably you signed up for the free trial and neglected to cancel before the 30-day period came to an end. The T&Cs do state that unless you cancel, it will automatically tick over to charged subscription.

https://www.ebay.com.au/help/buying/buying-ebay-plus/buying-ebay-plus?id=4755

$49  sounds like ebay plus annual subscription.

You claim you have 'never sold anything on eBay'

Yet someone used the account you are posting with to sell half a dozen things in the past

That aside

Have you really been charged , or has a scammer sent you an email saying you were charged?

If it is real

Does it say it is for selling

Or is it for something you signed up to, ticking you agreed to pay the fees for doing so (such as eBay Plus or Smartycashback etc)

Other members can't just guess going off a question in the heading alone

Have you contacted eBay and asked them?