I use a card that allows me to trash 2 cards from my hand (like Chapel). I trash Market Square and another card. Are the cards trashed simultaneously or sequentially? Does either possibility allow me to use Market Square's effect? If so, where does the Market Square end up?
The cards are trashed at the same time, so if you are trashing Market Square, you would not be allowed to discard Market Square for its effect also.
You choose all the cards you want to trash, and then trash them at the same time; you don't trash them one-by-one. This matters for interactions with Dark Ages cards with on-trash abilities.
Also, from the Dark Ages rulebook:
When two or more cards are trashed at the same time, as with Count, first trash them all, then pick an order to resolve things that happen due to trashing them.
You can't gain a gold by trashing Market Square, only by discarding it. Once you've trashed it you can't then discard it. Consequently you can't gain a gold by using Chapel to trash it and another card.
The only way I can see to trash the Market Square and use its ability is by discarding it, reshuffling, picking it up again and then trashing it. This could be done using the Doctor with a bit of luck (or a very small deck):
- Play a Gold and a Silver
- Buy a Doctor for 5 (overpaying by 2)
- Pick up the only card in your deck and trash it (Doctor overpay ability 1)
- Discard Market Square from your hand and gain a Gold
- Shuffle your discard pile and place as a new deck
- Draw the top card (which turns out to be the same Market Square) and trash it (Doctor overpay ability 2)