Can anyone please let me know if this is okay?
Good question! It certainly isn't answered in eBay's used clothing policy: https://www.ebay.co.uk/help/policies/prohibited-restricted-items/used-clothing-policy?id=4281&st=3&p...
The intention of this policy appears to be to prohibit underwear etc. worn in "intimate" contact with the body. Even if washed.
However used bikini bottoms, because they are not worn under anything else, do not seem to be covered by the definition. There are plenty of used examples for sale on eBay, so unless the rules are changed it seems you can safely list them.
You could contact customer support for advice, if you want.