As the seller, it is your prerogative.
Funny, just last night I was at the house of a turner from my club, who has a 3520 with extension and the swing away. We were discussing the fact I have neither. The conclusion we came up with was it would probably be better to buy a full package, and sell my existing 3520 than shell out the money for those two accessories new. I'll add that I have the 3520a, so there would be a minor upgrade going to the B or C versions, particularly if well optioned, and at a good price.
Bottom line, as a buyer, I'd be much more interested in your machine WITH the extension, than just the lathe.