Bribe
This is sort of a kingmaker card, which is to say that a losing player can purposely shift a win from one opponent to the other. If, in a 3p game, you're so far behind you can't win, while the player to the right has a slight lead, you can shift the victory to the left-hand player by passing him Golds and Platinums. Worse, you can justify it by saying you're passing those good treasures to earn a nominal amount of VP for yourself.
I also have the question about how much strategy is really opened up by the card. Although the opponent may accept or trash any card passed, probably these few choices will account for 90+% of all uses of the card:
1. Player passes Copper. Copper is trashed.
2. Player passes Silver. Silver is trashed.
3. Player passes Gold. Gold is gained. (Early and mid-game.)
4. Player passes Gold. Gold is trashed. (Late game.)
Certainly you *might* see a Silver being accepted, but Silver so often doesn't matter enough either way, so you'd probably not want one if it meant someone else getting 2 VP.
Of course when other treasure cards are in play, the possibilities are more interesting, but Copper, Silver, and Gold probably still account for most of the treasures in your deck. That begs the question: Why not be able to pass ANY (non-victory) card instead of just a treasure card? It might be more interesting if a player has to choose to accept or trash a Steward or a Bishop or something. On the other hand, this worsens the kingmaker problem, as then you could pass Goons and Grand Markets.
City-State
Very interesting card! You might want to read
this thread about WW's Squire card, as a lot of the same issues will apply. A big one is that the card's power is very variable. But this slight difference from Squire solves the problem of the card being useless on a good percentage of boards, because if nothing else it's a Village.
Because of the variance, it's a difficult card to cost. You might have to go to $4, even though on some boards it's weaker than Village, just because on other boards it will be a very dominating card.
Samaritan
Not sure you need the "every other player" penalty. Silver is usually a pretty neutral card. Sometimes you want a lot, and sometimes you don't want any, but most of the time the addition (or removal) of a Silver doesn't matter a whole lot. Which is why it might be most often the worst thing Jester can turn up. Playtesting, of course, will reveal the ultimate verdict.
Sphinx
Why the cost restriction? Allowing the player to decide between the two options, regardless of what card is turned up, would expand the strategic space. I love the idea -- kind of an inverted Jester, which is itself one of my favorite official cards. I think it needs to go up in price, though, regardless of whether you keep the cost restriction or not. Each play of the card makes it either a Lab or a junk attack, both of which are too powerful for a $3 card.