I came across someone saying there shelf was on hold cause of the 50 credit limit. 

What is that?

Melanie posted this in response to that thread: (the Help! I can't cancel! thread)

The new credit transfer limit since mid-August: 

Please note that there is a limit of 50 credits/month for credit transfers between members (this limit does NOT apply to Friends of PBS or other donations, credit transfers to or from Games Moderators, or credits given using the Give Refund button). 

The limit does not require Barbara to have her shelf on hold, she just apparently doesn't want to send any more books because she cannot transfer(sell) more than 50 credits in a month.

To clarify Robin's post, it is only transferring to another member, you are not limited in the number of credits you can receive. I asked the Team that question specifically.