The bottom ability is cool, but confusing when there are multiple players trying to discard Tax Collectors to steal stuff at the same time. It happens in turn order, so there are rules to cover it. I guess you can see if it's actually a problem in practice.
The top seems weak. Reactions have to be weak, but "Gain a Silver" is just way too weak for a $5 card even with a reaction ability. The default ability should be better. You could even just add +$2 to the top. Maybe that's too close to Explorer, but probably it's fine.
I agree that the top is weak, as I said in the description. My worry is that if the top is not weak enough, this will become an auto-buy and there will be a clutter of Treasure stealing come end-game. I want the reaction ability to be more of a "hey, I'm falling behind and this will help me catch up" rather than a "cool bonus to a good card". Fitting with the Flavor, the card is meant to be taxing on the players in the lead (if you're buying green with big money, you have to pay taxes).
My other thought is to make this a one-shot reaction. If I boost the top part (by adding +$2 or making it a Gold gainer), I could hedge the reaction clutter by making it trash itself to steal the Treasure. Thoughts?
I don't love the top, but I understand that it's hard to come up with simple tops for Reactions. The bottom wording seems awkward, although it's a cool ability. How about, "When you would trash a card, you may instead reveal this from your hand." I think that's sufficiently clear. I'm not sure the reaction works with all existing cards rules-wise, but no problems are jumping to mind.
You're bang on with the top part. It is difficult to make a simple top. This one started as a Broker, so I had the top similar to a peddler or market to fit the flavor. When I changed it to an Executioner, I tweaked the top a little, but not much.
I've been thinking about making some self synergy by having the top part a trasher, but again, I don't want to make it an Attack-Reaction. Perhaps I could get away with an "Each player trashes a card from his hand" top, but I'd be treading into dangerous territory if that's not labeled an attack. Maybe, "Each player trashes a card and gains a card costing up to $3" or something, then you could react to it with a copy of it and still gain the card.
The wording I took right from Trader, just replaced "gain" with "trash" and "Silver" with "nothing". I thought it was pretty clear, and shouldn't cause any rules problems (that's why the "if you do" clause is on so many cards!)