Here are some facts:
1. You will find a Sadar Bazaar in almost all North Indian cities which at one time had a British Cantonment office.
2. The word Sadar does not mean anything in Hindi or its variants
3. Sadar Bazaar is always located to the south of the city Center (unless the city center has moved over the decades)
Well a friend of mine has an interesting theory. He says that in Pre British era, the Britishers usually preferred to live to the south of the main city. The market that caters to their needs was called Southern Market. This market because of its elite clientele was the posh market and in many places still is. However the locals could not pronounce the word Southern and ended up calling it Sadar.
I could not find a credible online link to substantiate this theory, but it seems to be making sense