I don't like an early Market Square at all. If you commit to trashing, you want to start gaining gold only when you're deck is really trim, before that the Gold only sits in the way, not to mention that your ideal 4-card starting hand (after an opponent's Urchin attack) is Crossroads, Mercenary, 2 crap cards, so there's no room for a Market Square until you have your draw going.
I think apart from all the Silvers, your biggest mistake was not getting an early Crossroads and not focusing on Grand Markets. I'd definitely get a Crossroads over the Silver and probably even over the Potion. If you go for Mercenary you really need some sort of Village to support it.
My general gameplan would be:
- Get to Mercenary as soon as possible
- Add a Crossroads
- Get a Potion for $4, Market Square for $3
- Trash trash trash and buy Alchemists until you can draw your deck each turn even in the face of discard attacks
- Gain 2 Golds
- Start piledriving Grand Markets
I wouldn't buy a single Silver, unless you are already overdrawing your deck. Once you run out of Mercenary fuel, buy 2 Coppers each turn with spare GM buys. Also skip Black Market, at least until you have this whole Alchemist/GM thing running, and by that time you won't really need it anyway.