Vinyl marketplace, anyone signed in can edit anyone else's listing

You almost certainly wrote the update policy with only using and no with check. That is the single most common RLS mistake.

using decides which rows you are allowed to update. with check decides what the row is allowed to look like afterwards. With only using, a seller can take their own row and rewrite seller_id to someone else, or in a policy written as using (true) touch anything at all. Write both halves:

create policy "own listings" on listings for update
  using (seller_id = auth.uid())
  with check (seller_id = auth.uid());

Check your insert policy too, since insert only has with check, and a missing one there lets a user create a listing already owned by someone else.