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One such market is the Internal Market:
https://steemit.com/market

Click on Steem Back Dollars in your wallet and in the drop down menu one of the choices is market. It is an exchange with people selling their steem for steem dollars a little cheaper. But, this is only with steem dollars

So the roughly $12.00 SD I've put into conversion to Steem over the last few days would have been better put to use just going to the market & buying SP?

Yes

OK, live and learn.

It's not like I flushed it down the toilet.

I can still use the Steem once it's converted (after 3.5 days) to buy SP, I just won't get it at as good of an exchange as if I'd gone straight to the "Market" tab?

Reminds me of when I bought silver at $29/oz.

Now you know! That is all that matters! Good Luck to you!

You can sell your SBD for steem there is a market option in your wallet that tells you what the price they are selling at there and that's also where you sell and buy between the two. I only found it recently so don't worry about it.