Tamron 2x Tele-Converter for Canon camera

Tamron SP AF Tele-Converter 300F-CA 2x

I have a used 2x Tamron Tele-Converter I got with a 150-600 Tamron lens for birding.  It list for $159 used at Amazon.  I paid $150 for it.  My primary purpose for purchasing these together was wildlife photography, but I think this combination is pushing the lens capability to much. I have found that the Autofocus doesn't cut it at the higher end of the magnification.

The Tele-converter may due fine on other lenses.  This may be fine for scenery and moon shots.  I would recommend you read the various reviews on Amazon before deciding to purchase.  Because this isn't meeting my expectations I'm putting it up for $75 + 3% Pay Pal and shipping charges.