I think I like the idea for pirates, but I don't think it would work for players.  Typically planets are considered your most valuable assets by a massive margin.  I know that I'd never consider selling a planet to my enemies.  As well, there's too much room for dirty dealings, like quickly capturing a planet then selling it, and only moments later the original owner comes in to reclaim it from the buyer (perhaps even as part of a back-room deal between the two!).  It would be unfair to prevent him from doing so just because it was recently sold, but the seller just abused the system horribly.

You still need more specifics.  The biggest issue I have here is that this isn't a two-way relationship.  There are third parties floating around.  What rules are you suggesting to prevent dirty dealings by the seller and buyer?  How will you deal with possible back-room dealing with third parties without actually imposing limitations on unaffiliated third parties?  It seems to me this a very mucky place to be treading.  The concept seems nice, but I see practical quagmires across the implementation.

Aren't back-room dealings something that is inherent with the Diplomacy expansion?  Isn't the idea of it to make us think about who we can trust and who we can't?  Like if you suspect that there's something going on between two players, don't sell the planet to one of the players if there's a chance he'll sell it back to its previous owner.

And really, once a player buys a planet or anything from you, he's free to do whatever he wants with it.  If he wants to sell it back to the opponent for a profit, that's his deal.  And chances are he'd be selling it for a high profit on his part.  Unless he's just being nice.

Okay, let me express this as best I can.  I cannot forsee a situation where I would sell a planet under honest circumstances for anything short of a king's ransom, and chances are if anyone was wealthy enough to pay it they could sweep me aside by brute force anyways.  With this in mind, the only case I would sell a planet is if it's about to be attacked or as part of a dirty dealing with a third party to see it immediately taken from its buyer.

With that in mind, the buyer must PRESUME that this is some sort of rigged deal, and he will automatically bid next to nothing (if at all).  If he actually has the influence in the region to quickly defend the planet, he could take it by force anyways.  As a result, I think most of the time the deal would fall through or else would simply be taken off the market because no one is taking you up on it.  It seems to me that this is a lot of programming and extra rules (which will have to be somewhat convoluted to prevent abuse) for a feature that is likely to go un-used in most cases.

If it's with someone you supposedly trust, why do we even need the feature?  We already have tribute feature and abandon planet feature.

How about a Mission that lets you capture a particular planet within a particular time for a third party in exchange for resources, tech, ships, trading or mining rights, or what have you...?

Of course if you decide you want to keep the planet yourself and land colonists on it you piss the third party off to no end...

Currently the missions in game are rather boring, they could be much mroe interesting if tweaked however.

I see Darvin3's point, but hes neglecting an important part of Sins: Single player.

I agree that this would be considerably less used by human players, but I can see this as being of particular use when playing with the AI.

I'd love to be able to buy and sell planets with my computer allies, or buy a planet from an enemy. Sure, I could take it by force and probably have the military might to do just that, but it's nice to have options, and thats the point of diplomacy right? I would like more non-combat/non-aggressive ways of taking planets (using culture can take a while!)

Giving a planet to some other faction is not something to be done lightly. Reasons which would justify it:

• Streghten a relationship of allies
• Strategic importance
• Historical ownership (belonged to them before)
• Make peace

Some reasons combined would justify an end-war and instantly good relationship, like giving back an ancient, lost homeworld of prophercy to another empire to end a thousand-year war.

On the other hand, giving a planet just to rob it back is an act of extreme evil and deceit, which should carry a negative impact on diplomatic relations against all others. The player would be risking losing alliances, truces and much of painfully, slowly built relations because of a single planet. 

Players should be able to set their own prices for planets, after all it is their planet and their sale. Definitely agree with the destruction of orbital structures etc though.

A grace period wouldn't be necessary if there is a severe diplomatic downfall to betrayal as mentioned by Brazilian_joe, for example, a decrease in relations with every (non-allied?) player, unless it is an ally you've betrayed, if you do damage in that system within a certain length of time. The reason I say to every non-allied is because such a betrayal would be a strategic act on behalf of the entire team and would benefit them all, so there's reason it would not be seen as a dangerous move from the player's own team. Although I can see why it would build distrust.

Furthermore, a long grace period which stops selection of the planet would leave the planet very vulnerable if further research needs to be carried out on it.