If there are multiple possible triggers, then the rules say that you choose which order they happen in. So if there are two "when buy" effects - e.g. you've bought a $4 non-Victory card when both Talisman and Haggler are in play - you resolve both, in either order. If resolving one trigger nullifies the other (particularly in cases where the "lose track" rule kicks in) then you don't get to do both.
In the case of your trashing-Squire-then-Market-Square-and-Watchtower example, there are two trigger points - first, when your Squire is trashed, both Squire and Market Square can react to that and neither interferes with the other. Then, when you resolve each of those, you gain a card, to which you can react with Watchtower. Thus:
Trash Squire
1Gaining an attack card
1.Reveal Watchtower
1..Top-deck the gained card
2Reveal and discard Market Square
2.Gaining a Gold
2..Reveal Watchtower
2...Top-deck the Gold
Where the 1 and 2 blocks are interchangeable. So there is a little bit of a stack, but it's hard to make it nest more than this without a lot of very specific cards. One particularly nifty case is the interaction of Market Square and a trashed Cultist:
Trash a Cultist (with an appropriate trasher)
.Reveal and discard Market Square
..Gain a Gold
.Draw 3 cards with Cultist's on-trash ability
.Reveal and discard the Market Square you just drew
..Gain a Gold
Then presumably resolve the rest of the card that trashed Cultist
The trick is that the second Market Square is actually the same as the first one - if you have no cards in your deck when you draw from Cultist, you thus have to shuffle your discards, which means you have a chance of drawing back the Market Square you'd just discarded. It's the same trick as with Secret Chamber (which, when an opponent plays an Attack, you may reveal to draw two cards then put two cards from your hand on top of your deck) - you can do some screwy stuff if you have other "on-Attack" reactions:
Opponent plays Attack
.Reveal Secret Chamber
..Draw two cards, including a Moat
..Put two cards on top of your deck
.Reveal Moat
.Reveal Secret Chamber
..Draw two cards (the same ones you put back before)
..Put two cards, including the Moat, on top of your deck
But the important thing is to always remember what you're reacting to. Moat and Secret Chamber react to an Attack card being played. Tunnel reacts to being discarded. Squire, Cultist, Feodum react to being trashed. Haggler, Goons and Talisman react to buying a card, while Border Village, Watchtower and Ill-Gotten Gains react to gaining it, but Trader reacts to "would be gaining", a mystical stage that happens in between the two.