I am planning this for next year. So here I'd like to gather community feedback and interest.

To enable future peer-to-peer marketplace and other advanced p2p applications I am considering adding auxiliary transaction support for a major future release of ppcoin.

Main design point is

• Each regular block is associated with an auxiliary block.
• Auxiliary blocks are not permanent. They expire after some fixed time and are garbage collected by ppcoind.
• Each auxiliary block includes a set of auxiliary transactions.
• Auxiliary transactions also has expiration and may include a (key, value) data item in each output script.
• Auxiliary transactions also pay transaction fee.
• Auxiliary transaction output cannot be spent by regular transactions.
• Auxiliary transaction may spend regular transaction output only if it's a special pay-to-auxiliary type.
• Auxiliary transaction/auxiliary block is not allowed to mint coins.

The above describes the internals of a recyclable auxiliary block chain feature.

A potential p2p marketplace software that communicates with ppcoind may implements:

• Listings are inserted into block chain as auxiliary transaction outputs.
• Listing includes seller's cryptographic identity, item description and asking price. Output has amount 0 and can be spent by anyone with another auxiliary transaction. Spender effectively claims the listing (becomes buyer) and may encrypt his/her shipping address in the auxiliary transaction output.
• Buyer now makes payment according to listing instructions.
• Users leave rating/feedback via an auxiliary transaction for the target identity.
• Manage identity/reputation/web-of-trust.
• Index listings and provide search functionality.

Note this feature is a generic design and may enable other type of p2p applications, for example, a domain name system similar to namecoin.

Comments, suggestions and feature discussions are very welcome 
Also looking for any community interest in designing marketplace or other softwares taking advantage of this feature.

Strange suggestion,

Would it be possible to build anonymous-messaging on top of an Aux Chain/Blocks?

Messages sent signed with keys, encrypted with known GPG keys; could set max lenth of X bytes, might be a nicer messaging system than say TorMail; also as they expire after X time, it would allow the "client" to if so the receipient wants, "pull" the message off of the chain.

If the items you are selling are smart property they should not be garbage-collected as their whole point is the blockchain provides a permanent record of ownership. If they are not smart property you might as well put them on your website and just put directions to your website in the blockchain (thus likely use namecoin to do it).

-MarkM-

I did not mean pull as in remove, pull in as far as make a copy for themselves, maybe in their wallet file. Any machine, aslong as you have your wallet keys, you could read your recent mail. The 1c/KB/message I think works fine. Most "messages" arnt much more than a few kb's. Obviously that could be changed in the future.

Just throwing out ideas. 

Ah I see. Although the application software is meant to manage their own copies so these messages won't be stored in wallet. Yes as long as you have identity keys you can always reconstruct a copy of your messages that have not been garbage collected. This can be done by the application via commands similar to getblock/gettransaction.

I don't know if it's a good idea to reuse wallet keys for identity keys, or if there is requirements for wallet to store a different type of keys for identity.

Sounds great so we can touch base again when you have more concrete plan and requirements 

I messaged Sunny about this a while back 

Would be interested to hear any more thoughts of his on this. I know he is busy strengthening the proof of stake side of the network (I believe).