...* Always use messages signed with your address and ask the same from the parties of the deals. Make sure the parts are aware that only such signed messages are meaningful. Impersonation is a big risk in your business. ...
Newbie question: What kind of messages does this refer to exactly, do you mean messages and PMs on this forum? If so, sign how, is there a SMF or forum specific mechanism for that, and how do I prove my public key, if it's going to be safer than my forum login?
Sorry for my ignorance
I've said it, but it seems that you've ignored the most important part. Bitcoin allows signing a message. If you own a bitcoin wallet you can sign a message with one of the addresses of that wallet (actually, better said, with the private key of such an address).
So best is that all parts of the deal have bitcoin addresses staked (you'll need to learn to sign a message, you'll need to also take a look onto this thread: https://bitcointalk.org/index.php?topic=996318.new#new ) and sign their deal related messages from those addresses (maybe also stating "current" date/time).
So no, it's not a SMF mechanism, it's a Bitcoin mechanism we also use for proving ownership of bitcointalk accounts in case of troubles (this topic can be informative too: https://bitcointalk.org/index.php?topic=5436734)