Tough roast.. what's going wrong?
Meat is tough for two reasons:
1- An abundance of connective tissue.
2- When over cooked. 

In your case I'd say you probably have both problems. Cheap meat is tough meat. It is from older animals or well worked muscle groups. This means that it has been fortified with a lot of extra connective tissue. It also means it has a lot of flavor.

The solution to #1 is slow, wet cooking that will melt that connective tissue into delicious gelatin. Braising is the normal way to do this. When meat is heated too far, even if the connective tissue has been carefully melted out, the meat proteins bunch up and stiffen- resulting in #2, a dry, unpleasant meal.

You are buying tough meat and cooking it relatively quickly with no thermometer. You don't have enough time or moisture to melt the collagen and you can't be sure you haven't over cooked the meat because you don't know the temperature. Using time doesn't work because chunks of meat are irregularly shaped so you can't know how long it will take for the heat to penetrate.

Buy yourself a probe thermometer to prevent #2. For #1 look for pot roast recipes. Some are easily done in slow cookers- others use a tent of sealed foil over the meat to seal in moisture. Plan on it taking much longer than your 1.5 hours. 3-6 hours are typical to produce a really succulent pot roast.

The searing is just for flavor and will not play a role in either melting collagen or cooking the interior of the meat.