If you want to set shop in Pixel Gate. You can set in an empy plot, or set a 1 plot shop near the centre of the city, asking @Karokendo to help you with it. Only requirement we ask in the one plot shop is for it to allow access to the neighbors. On the shops by the roads, we only ask for it to look good; yes it's a very wide concept, but truthfully it's hard to go wrong in Boundless with the nice blocks and colors.
On the Therka Market I remember hearing one time, that it was 2 blocks on the side of the road and then all you wanted up or on the sides. Can't really give specifics as I haven't set shop there. You can ask @Heureka for further help.
As on Berlyn, it's free entry. Don't know if they have any wishes or guidelines, @Havok40k can fill you in better than I can.
All places mentioned above, have free entry. But I don't know if they have any preferences to go about them, except for Pixel Gate.
Hope to see your shop soon.