Oh how I miss Marshalls and Costco! There are a few places you can try that I know of...
One is Easy.. (www.easy.com.ar) they are like a cross between Target and Home Depot. Jumbo (www.jumbo.com.ar) is another alternative, that is somewhat of a KMart type place. There is a shopping center that has both, I can't remember the name right now. The area it is in is kind of unsafe, but if you drive fast to get to the gated parking, you should be good.
The last and most interesting in my opinion is the Mercado Central (http://www.mercadocentral.com.ar/site2006/index.php). The feria part is open Wednesdays, Saturdays, and Sundays (the wholesale fruits and vegetables are open daily). The general purpose of this place is to provide wholesale fruits and vegetables to the stores, but other than that, it has a HUGE feria with just about anything and everything. Quality of goods is sometimes an issue, but I do remember seeing some large shops that carried almost all housewares, and looking pretty nice, and cheap too! Good luck!
in addition to mercado libre, try the auction houses particularly Naon on Quintana. Good bargains can be had. Don't overloiok the mercado de pulgas on Dorrego and the various anitque stores along on the 7500-8500 blocks of Av Rivadavai, e.g. La Rueda.
Try Vital, it is a bulk store. I´ve found good bargains there. You need to join, might need DNI, not sure.
Normally offers savings of about 25% but you need to calculate, not everything is a bargain. Pharmacy stuff is much cheaper, especially fem hygine, sunscreen.
kind of like a Sam´s club, but ony food, no electronics.