Two parts: 1. The manager would probably get in fairly deep shit if he gave you a $35 cable for $10, because even they have to answer to a district manager, and discounts like that are looked at pretty heavily.
2. If Best Buy is anything like Staples (where I currently work), then the workers fall in to one of three categories: a)kid trying to work through school. b) guy who just wants a job, and will do absolutely nothing to improve at the job beyond what is demanded by management. c) those who enjoy the job, want to stick with it, and would like to move up the company. Now, training falls in to two sections. The online product knowledge modules that talk very generally about things, and are often a couple years out of date, and practical training on the job by co-workers. Types a and b are unlikely to remember much about the online training, and tend to get things wrong, and c isnt' a terribly common type, so most co-worker training is done by a and b types as well. Hell, I had a customer in looking for digital projectors, which I knew shit-all about, and he was kind of pissed when he figured that out, but then we ended up spending about 10-15 minutes chatting so I could learn more and not be such a dumbass.
The problem is that places like Best Buy and Staples have a fairly high product turnover rate for things like TVs, computers, etc. so learning the specifics of every product would be a huge resource investment, most manufacturers don't care enough to provide companies with training modules or materials, and don't pay enough to motivate most employees to do independent investigation.