If teams are already selling contracts to Japanese teams, a trade with a Japanese team has to be possible.

No it isn't. Not under current rules, at least. Players who want to play in the other country can have their contracts sold, but there isn't a mechanism for trading of player for player - and I doubt that either league wants to develop such a mechanism at this time.

If teams are already selling contracts to Japanese teams, a trade with a Japanese team has to be possible.

Aren't these contract sales contingent on the player agreeing to go? It's not like he has to retire or report is it?

No young American prospect would agree to play in Japan that was already playing in the US minor leagues, unless significant money was involved or unless the player was not "young" and instead was actually old for his league or a quadruple-A player.

Most minor leaguers Japanese teams would consider are old for their league, though. Japanese teams want someone whose performance will be predictable and who doesn't need any more development time; since they're using one of the valuable gaijin slots on the guy, they want to know he can fill a role.

Both Steve Fireovid's book and Chris Coste's book have passages about how when they got to be AAAA players in their late 20s, they started thinking their best hope was to draw attention from Japanese teams.

Yeah, how the heck is Fredrickson's name mentioned here?
The guy, for all his issues (controooool) was just a first or second round pick in 2008. Hes not going to Japan.

Plus, as post 5 points out, he wouldn't be all that useful to Japan anyway since his pitchability is so poor right now. Him on a Japanese League team would be like making him a Rule 5 pick

Darvish is part Iranian; would that affect his popularity with American fans? Would he become baseball's answer to this guy?

"If teams are already selling contracts to Japanese teams, a trade with a Japanese team has to be possible."

<< No it isn't. Not under current rules, at least. Players who want to play in the other country can have their contracts sold, but there isn't a mechanism for trading of player for player - and I doubt that either league wants to develop such a mechanism at this time. >>

That whole statement is messed up.
Under union rules, a major league contract is only assignable [sold] to another major league team. (Obviously the player can negoiate more restrictions placed upon that.)

If we are talking minor league contracts, then the author describes the buying out of a contract with compensation to the player, compensation to the previous owner [sold] and a new contract agreed between player and team under Japanese League rules. The writer might call the entirety, a 'trade', with the compensation to the previous owner being a similarly compensated Japanese player and an agreement to play for the MLB team.

So, in response to Ivan:
All players would have to agree. The owner of the player's current contract would have to entice the player to quit [void] the current contract in favor of a new one with the new team. If the owning team simply decided it didn't want the player any more, it could either honor the contract or reach an agreement with the player becoming a free agent. There is no mechanism in place to exchange and recognize the contract of a 'foreign' league.

In any case, it is more correct to think of it as rights with new contracts being exchanged rather than players being exchanged.