During a Duel, all actions taken by either player must be declared and acknowledged by both parties
Pokemon GO has a tricky little hidden mechanic in it some of its cards are secretly a ditto
Both games have official formats that define the card pool that players may use when building decks, complete with ban lists and the like
For expensive cards, use the app as your first pass, then confirm the exact variant, condition, and recent sold comps before listing, trading, or submitting for grading
If the only monster you control is 1 Level 5 monster: Tribute it
Note that Toon Cyber Dragon's name doesn't become Cyber Dragon like other Main Deck monsters in the archetype, so monsters like Chimeratech Fortress Dragon can't be exploited this way