Cheryl wrote: I don't think Avon was sent to Cygnus Alpha for any crime other than an
attempt to defraud the Federation's banking system...<
I agree. In Space Fall Vila only mentions the bank fraud, not the dead visa seller. The Ultra also only mention the fraud. This leads me to believe that Avon was only convicted for the fraud, not for the killing (which, a capable lawyer could argue, was in self defence as the visa seller shot first).
Possibly the authorities a) didn't regard the killing of the visa seller as bad as the fraud, and decided to prosecute Avon only for the "graver" crime; b) were actually grateful to Avon for ridding them of the visa seller, thus sparing them the costs for a trial; c) didn't even know that Avon had killed the visa seller.
Anna didn't know where Avon was to meet the visa seller, or she could have directed the hounds straight to that place to pick Avon up. As Avon was only caught some time later - he was at least a week in hiding and we don't know how long on the run after that - it is possible that he never was connected with the killed visa seller. Or maybe the seller's body was never found. Or the authorities had already convicted someone else for the deed and thus couldn't prosecute Avon for it without having to admit their error.
Marian