So I sold TV's at Best Buy over the summer when I was home from university and I can tell you what I learned, so first off on a 55 inch tv the markup isn't as huge as some people believe, sometimes it's like $30 (mostly on sony TVs, the ones I sold always had no markup or tvs that have been out for a bit and the price has been lowered over time) and on the higher end it can be $500 max (on the highest end tv we sold), 80% of the time it's $100-$200. Samsung 55inch smart tvs are decent and have a $200 markup typically.
My store would not give a discount if all you were buying were products and since the staff is non-commission it doesn't really make a difference to the staff if you say you'll only buy if you can get a deal but it also means that they're willing to discount more since it won't effect their paycheque. Realistically you'll only get a discount from best buy if you were going to buy the service plan, or were looking to get something done with geek squad (we sometimes just throw geek squad services in for free if the tv had a decent markup and we needed to bring more geek squad business in), otherwise with just products you have a 1% chance of geting a cable thrown in at most.
If you were getting a service and you wanted a discount, ask for it upfront and make a lowball offer, the more products you are buying the larger discount you could get on services.