The software does have some good points. It does simplify listing on eBay for the small to mid-size store and makes shipping much easier than the shipping module in eBay. That being said, the software is very buggy.
Many times it will not list all variations
Installing updates invariably creates more bugs
Revisions have to be sent multiple times to get them to post
Reports error messages when there are no errors
Customer support is horrible
I have been using the software for nearly a year and I am having some of the same errors that I reported during the first month. I open a ticket, they say they don't see any problem and close the ticket. You can't call them unless you are willing to pay additional to talk to someone.
My advice is to use something else that is more reliable. I haven't found what that is yet but don't use SixBit.