http://www.lenntech.com/processes/disin ... lorite.htm
Take a bottle of your bleach (buy generic, don't buy name brand Clorox unless that's all you can find. Where do you think Clorox got their name? Sodium HypoCHLORITE = CHLOrine), and show them the ingredients on the bottle. Typically most brands and generic bottles of bleach have one or two lines for ingredients. One will show sodium hypochlorite. See if the pool store sells any liquid shock or liquid chlorine. Look at the ingredients. It will be the SAME THING.
Note this is comparing liquid shock and liquid chlorine. Powdered shock products are calcium hypchlorite and other varieties. Same base but has calcium and other additives which we don't want or need (unless you have a plaster pool and know that you need it). The reason we tout the BBB method is we want to add ONLY the single individual items that need to be adjusted. Pool store chemicals are full of everything under the sun because they don't want to encourage people to learn how to maintain their water properly. They say 'stick this strip in the water and guess at the color and throw these tablets in full of 'catchall' ingredients and you're done'. That's why after the end of the first season and later when so much CYA, calcium and other things get to be too high, people get frustrated because they can't keep a clear pool.
Tell them sodium hypochlorite you buy in the store is typically 6% concentration. If a pool store sells liquid shock it's usually 10-12% concentration. So if their 10-12% concentration won't hurt liners then how does 6% of the EXACT SAME CHEMICAL hurt it?
I'd be upfront.... print that article out or one of hundreds of others online, and tell that person they really shouldn't spread grossly inaccurate information just for the sake of trying to sale high priced chemicals.
Also, you probably paid a lot more for muriatic acid there than you would have at Home Depot or Lowe's.
CYA is usually the only thing a pool store has that we need. Of course there are exceptions, but we're talking basics right now.
As far as convincing you... look how many people and how many YEARS these people have been using bleach in their pools. Again, bleach is nothing more than a common name and in lower concentration than what you buy when the bottle says chlorine or liquid shock. Nothing more.
I'm definitely not a veteran around here or expert so I may get corrected on some minor points but this should cover the issue for the most part.
I would absolutely print that page out and take it back and just lay down on the counter and politely say "you really should read this and understand what bleach actually is before giving out incorrect information".... or better yet give it to the manager and tell him/her you wanted to support the local pool store but find it hard when you're told information that's basically a lie. That way you won't offend the cashier to their face.