You put 1 item into a repeat block with this:
set mailboxList to mailbox "INBOX" of account “MyAccount"
repeat with theCurrentMailbox in mailboxList
You can try something like this:
set daysToPreserve to 2
set myAcount to "MyAccount"
set dateReference to (current date) - (daysToPreserve * days)
tell application "Mail"
activate
set myMailbox to mailbox "INBOX" of account myAcount
set accountTrash to mailbox "Trash" of account myAcount
set messagesToDelete to messages of myMailbox whose date received ≤ dateReference
repeat with aMessage in messagesToDelete
move aMessage to accountTrash
end repeat
end tell
display dialog (count messagesToDelete) & " old Mail Messages Have Been Purged" as text buttons ["OK"]