Trap 3 — the code you can't get a copy of
Even with a one-off build, ask explicitly: do I receive the source code, and is it built on open, standard technology I could hand to any other developer? Some agencies build on locked, proprietary builders that cannot be exported or maintained by anyone else — so even though you 'own' the site, no other developer can touch it, and you are effectively tied to the original agency for every future change.
This is where the underlying technology matters. We build on standard, open technology and hand over the source code on delivery, so the site can be hosted anywhere and maintained by any competent developer — including us, but not only us. That is what real ownership looks like: you are free to stay because the work is good, not because leaving is impossible. Before you commit, ask whether you will get the code and whether it is built on something a third party could pick up.