I often paint objects people things landscapes etc on a canvas upside down. When I display and or sell it I flip it the other way so they are all the right way up. I'm just curious if this technique is common? It's probably not normal but what in art is normal anyway, right?

Hm, but to me I do think it's normal. I for one draw sideways and sometimes draw in portrait a landscape picture and vice versa. Painting upside down is a bit unusual but doesn't really count as abnormal.