Let me understand correctly,
You are new to BTC but you have unlimited amount of BTC to sell..legit.
if you want to post TXID you have to get it from your wallet or go into the blockchain with your address example:
https://blockchain.info/address/13XrHSo7XKGAsKSgL4qzNmKWZMUjmD34Lq and then txid will be next to the date time of the transction for that example, last transaction txid was
d3d4058ca1a52b6aed8f8ce2d27df83fbf1bde9a8b57d79546e3cd9f635edd38 and date was (Fee: $ 0.04 - Size: 226 bytes) 2014-10-12 08:56:09
To sign a message, let us know which program do you use to manage your bitcoins so we can tell you how to sign messages proving that you own the bitcoin first.