I was a bit sad that overpay did not reappear in Renaissance, it is not a difficult mechanism and there is stuff you could do with it and the tokens.
Marketeer is simply another name for a Buy token (that you can spend at the start of your Buy phase for +1 Buy); overpaying for VP tokens obviously doesn't work (on a Victory card, this card by Theta and AdrianHealey does work).
The anti-Copper clause is there to prevent endgame shenanigans with total Coin transfer; I don't like the enusing overall wordiness though.
Printing Press could be good enough to cost $5 and could be too similar to Academy, Guildhall and Spices.
Yeah, I think this is pretty strong, especially considering the Coffers for the non-Copper Treasures. There are many instances where buying a Silver for 4 or even 5 is a good move without the Coffers bonus, and let's not forget $7 or even $8 Golds. In fact, I could see myself going for a Silver for $6 and taking those 3 Coffers over the Gold easily. As you said, this can compare to Spices, to the point where Spices is almost obsolete in Printing Press games apart from the +Buy. I like that you are bringing in Marketeers, though. More token uses!
I always liked the recent Actions that want Silvers (Sauna, Merchant) or Golds (Encampment, Legionary) in your deck so this is my stab at doing something along these lines:
Both cards are part of a split pile with either 4 of each in a two player game respectively 6 of each in multiplayer.
Timber Raft could be good enough to cost $3. The base price of Riverlands is just a first shot, hard to determine this without playtesting. Riverlands could be in principle too strong. If you manage e.g. to play 3 Timber Rafts and 3 Treasures the first one costs only 1, the second one 2 and so on. Twitching the base price only gets you so far which is why Riverlands is then up for elimination. Its idea is mainly to make Timber Raft better but perhaps this is already good enough on its own.
By itself, Timber Raft would probably be okay at $2. Apart from Goons, you rarely need all those Buys. Also, you need to reliable get those non-Coppers in your hand to make it work, so Timber Raft doesn't become all that good until midgame at least.
With Riverlands, though, you are right that this could get shenanigan-y really fast. In your situation with 3 Timber Rafts and, let's say, 3 Silvers, it would take just $10 to wipe out the Riverlands in a 2-player game. You have $6 already, and with a decent engine, the other $4 shouldn't be too hard to get. This then becomes effectively a race to raid the Riverlands... Never mind, I love it! Alt-VP for Life!
In any case, make sure to add a "but not less than $0" clause to Riverlands.
Moreover, the whole idea of a split plot with 4 or 6 of the non-Victory card is just weird, and I usually like weird. Is it truly necessary for Timber Raft to have 4 or 6 cards in the Supply rather than 5?