Our single user license cover two computer if you are the only GarageSale user. No need to purchase an additional license.
You can, but since there is no way to export or import reports you would need to re-create identical reports on both Macs manually.
Yes, but that may be cumbersome to achieve. Both Macs would need to know about all listings started on the other Mac, which means exporting/importing after each listing start. You could also use the “Import from My eBay” command to import listings that were created on the other Mac.