Are they similar to a Tesco superstore?
Be warned, Marktkauf on a Saturday is horrendous. Casino is not so busy.
Best IMO is the Leclerc Drive in Saint louis http://fd7-www.leclercdrive.fr/default.aspx
Simply do your shop online and collect in a forecourt. Someone comes out with the shopping and helps pack your car. Fast and efficient.
I recommend Lörrach as well, it is covered by the tri regio mini ticket (24 hours for 7 euros or 10.50 sfr), there are 2 supermarkets from edeka (one at Museum Burghof and one at Lörrach HBf) and there are more stores in Lörrach itself (from Karstadt to Douglas, DM to name a few),
see yourself at google maps.
The tri regio mini ticket takes you to ST.Louis, Pratteln, Bale and Bartenheim(SNCF)
Go to E.Leclerc just the other side of St Louis. It takes me another 10 minutes to cycle there so under 5 in the car - the bus from Schifflander also goes there.
Marktkauf is cheaper for almost everything (and getting tax back is easy), but the fish, deli and bakery products in E.Leclerc are miles ahead of anywhere else, including Hieber (which imo doesn't deserve the good reputation it has - Marktkauf is part of Edeka after all, which is another "premium" German supermarket).
If you're taking public transport and don't need the wide variety of non-food stuff that Marktkauf has (I buy all my cleaning products there; about 1/3 the Swiss prices plus 19% tax back) then stay on the Tram 8 and go to Rewe in Weil centre.
You can buy it for 7€ at the German machine next to platform 1 at SBB. Not sure why everyone doesn't do that, but sometimes it seems I'm the only person who works out these "tricks". Does no-one else care about money?
so better come prepared(Coins !).
Another options are the station Basel Bad Bhf /German Station of Bale or the Busdrivers of the Distribus , or the vending machine of the sncf.
have a nice day