I've searched in the help area but cannot find anything conclusive about this.  Is it really okay to sell credits?

Yup!

Hi Oney,

From the Help Center:

Can I sell my credits to other members? Yes.  While we do not "buy back" credits ourselves, you are free to sell your extra credits to other members or give them away.  You may set the price and payment method.

To offer your credits for sale:

• Start a Discussion Forum topic explaining that  you have more credits than you can use and would like to put them back in the system.
• Ask interested members to send you  a Personal Message asking for more details.

To obtain payment:

• You need to work out the payment method between you and the other member(s).

After you have been paid by the purchasers for the credits:

• You can transfer the credits using the Buddy List Crediting Feature.
  • Read instructions on how to use this feature here.

 You can post your offer to sell credits in Book Bazaar. 

I would consider buying credits from another member.  I have bought some from here already, while I'm waiting for my credits to accumulate from the books I've sent!

It happens all the time in the Book Bazaar. I've bought quite a few from people offering over there.

I've sold credits twice to people who posted on the book bazaar.  Not many but it helped cover postage and I have a lot of credits. 

Well, ain't that dandy!  Thanks for the info.