First thing that comes to my mind is that you need to mark those baskets very clearly somehow that they are yours not the stores. Otherwise things could get potentially interesting when you try to "make off with them", especially true if they are the same color and style as the ones the store is using. In my experience when I go into a store and grab one of those hand baskets and fill it up and put it on the cash register belt they bag the groceries and hand them to me in bags and don't hand me back the basket. If its your basket and it isn't marked as yours very clearly that could be a point of potential conflict.
As far as the Wald rear baskets go if your bike is a conventional hard tail bike and your mechanically inclined mounting isn't too bad although you will still probably have a few bolts, washers, nuts, and assorted brackets of your own to supplement what is provided. Now if your bike does not follow the standard hard tail frame configuration mounting could get a little more interesting. So far I've been able to make them work on everything I've tried to mount them too but some projects have been full day projects with a lot of fabrication and ingenuity required.