As lachlanandmarcus said plus you need to keep your feed sack labels for 3 months. You need all your receipts from the slaughterhouse, butcher etc - the tags which come back with each carcass will do.
Carcases are stamped at the abattoir so that after jointing the stamps can still be seen.
I have sold meat in the past and had it cut and packed by a butcher, to conform with regs. I have also bought a wild boar carcase and collected it uncut from the abattoir, but not to sell on. If you are going to cut meat at home then sell on the joints, you have to be inspected by Trading Standards and conform to regs. Before those laws came in we were able to collect the carcases from the abattoir then have them cut up in our own kitchen.
I think that if you sell meat from a freezer then there are further regs about that - dedicated freezer, use-by dates and so on. Best to check with Trading Standards.