By requiring them to be dual-type, you are needlessly complicating and adding potentially unwanted interactions to the simplest of these cards (i.e. the ones that just offer something for sale). All of those interactions are still possible where Supplier is its own type (by making it a dual-type card) without forcing the complexity on the cards that don't need it.
I also agree with the two points laid out by Timinou. Having an entirely new card type with new color and all is far more needlessly complicated than a dual-type. Its the reason why Looters is simply a secondary type instead of an entirely new category of cards. Nights were pretty simple as far as a new type category goes, and yet Nocturne is widely considered the most complicated expansion, partly due to that mechanic. There's no reason why the simplest of the cards you mentioned can't be simply dual-typed treasures, that is a far simpler approach than the rules laid out in the op, and it seems that sentiment is agreed upon unanimously.
What makes a new type more complicated? You still have not explained that. How are dual types cards less complicated? Which of the instructions in my original post wouldn't need to be there using the dual type? As far as I can tell, all of those directions would still be necessary.
I've explained multiple times that with many Supplier cards, if they were Treasures it would create many interaction with existing cards that are both thematically illogical and practically undesirable. That's not universally true, and I think your Bribe or Timinou's Fare make sense as Treasure cards. But it will never make sense that a Bandit steals a Summoner or a group of Cossacks.
Looters are all Action cards, each of which only does things existing Action cards do (+$, +Cards, junking, trashing, gaining, etc.). It would make no sense for them not to be Action cards. The only reason they have a separate type is to indicate to players to add a Ruins pile.
If Nocturne is complicated, it has at least as much to do with introducing Boons, Hexes, Heirlooms, Spirits, and Zombies all at once than it does adding Night cards.
The sentiment isn't unanimous, since I still disagree with it, and I have yet to hear an actually explanation of why a new type is more complicated than a dual type.
This is incidental to the overall contest and discussion, but Miners and Ranch are incredibly weak. Here is a card that almost exactly replicates the ability of Miners, while being strictly stronger than it, yet it is still quite weak:
This lets you get 2 silvers for $4, a silver and gold for $7, and 2 golds for $10, without being nearly as restrictive as Mine.
And Ranch could just as well have stated "You may pay $2 to have this card be a cantrip later, or $4 to be a lab". It goes without saying that a one-shot cantrip that costs $2 is worthless, and one-shot lab for $4 is significantly weaker than both encampment and experiment.
Timinou previously made the same point, and I previously addressed it.
For example, I don't think Ranch really adds much over Ride to justify having an entirely new card type.
You already mentioned that the value proposition of Suppliers needs to be much better relative to an Event, given that Suppliers could really end up being a junk card in your deck. Of the examples you posted, Miners and Ranch look quite weak.
I said these were examples, I didn't say they were good examples. I mostly wanted to show people see what the cards looked like, as I felt the explanation might not have done that in the most clear way. I agree that both of those cards are rather weak (although I think Miners could be useful on some boards). I put Ranch out there as a suggestion of how one might turn an Event into a Supplier (but I agree that it is not enough of an improvement).
Again, I put those out there to show how the cards worked mechanically.