This is a typical problem for this series Viking. It's also usually not an easy fix. These machines basically need heavy duty servicing that entails removing all the old grease that has now dried to a wax paste and has caused either the buttonholer to get out of alignment and causes the machine to be stuck in reverse and or a complete resetting of the buttonhole cam inside the machine. Typically the dials are stuck and get twisted and can break no longer available parts inside the machine.
The camstack may have cracked and the take up slide needs replacing if it's an original part. Each of these parts cost $40-$50 each. Labor NOT included.
Many people won't tackle these machines due to the time involved. If you go to the vintageviking group on yahoo, there' s a listing of trusted repair people. Many repair shops throw some oil around inside the machine and call it a day, but there's quite a lot work that needs to be done properly to keep these machines around. I've rebuilt two of the 6000 series machines and can tell you that working on these machines isn't easy or quick.
Ebay machines are sold and the buyer gets to deal with all these issues. They're great machines when an investment of time and money is made. But, I would bet any machine that is not fully operational and is sold on ebay will need at least $200 of parts and labor.