Unless you've got a side-car, a Dob of any size will be awkward on a motorbike. Have a look at the following option which has its own 'mount' but you need a table top to place it on for a more convenient height, or just hold it as it will work well for scanning the sky at low power.
http://www.scientificsonline.com/ast...telescope.html
This telescope has been around for over 30 years and the new model has a few good improvements.
I would recommend either the above telescope or a small refractor for portability but you still need a mount for the refractor although you could get away with a camera tripod.
As far as having separate classifieds for beginners, all you would be doing is creating a separate list or lists of equipment for sale and you cannot really stop non-beginners buying from it nor should you. There are really good bargains at times and at other times not so. Just keep looking and you will eventually find something you like.