First off, it's too bad that anyone has a bad experience with a dive shop. This sport is supposed to be fun, and situations like this just suck the fun out of things. I have to agree, however, that EVERY certified OW diver should know how to do a buoyancy check and SHOULD do one every time they dive in a new environment, with new gear, etc. If a new OW diver doesn't know how to do this, then that is a failure of the instructor and the certifying agency, IMO. BUT, and this is a big but Simone, ultimately everyone who undertakes scuba is responsible for themselves. No matter what good or bad advice one is given, in the end, each person is responsible for making sure that they have exactly what they need and know exactly how to use it.
If the shop sold too much weight, I see no reason why they wouldn't gladly take the excess back as a return or exchange. It's not like lead weight has any working parts, etc.
As far as the reg goes, if my primary reg DIDN'T free flow at the surface, I'd be concerned, but in the same token, I can always stop it with a thumb over the mouthpiece or turning it upside down in the water. It sounds like this reg could not be stopped from free flowing. As has been mentioned, checking it before you leave the shop is critical. Yes, the shop should do the same, but I'll refer back to my previous big but Simone; ultimately, we are all responsible for ourselves and unless under the care and feeding of an instructor, should be taking all the steps necessary to make sure that we finish the dive alive. Blaming the shop for not checking in no way excuses the diver from not doing it themselves.
Hopefully, msscuba will be able to do what we all should do from situations like this and learn something new. Buoyancy check in every new environment and new gear config, check rental gear before you leave the shop, and the biggest....In the end, it's your life, depend on you first before anyone else to make sure you have the right training, gear, and buddies to end the dive alive.
And hopefully EZ Scuba will read this thread, and at the very least offer a refund for the excess weight and a free future rental. It's good customer service and will keep people coming back. Let's face it, times are tough and dive shops are hard hit these days, I can't imagine why any shop wouldn't want to make every customer a return customer.
Edit: Oops, a lot happened while I was writing this. Looks like EZ Scuba has been in contact with msscuba. I see no reason why Eric owes all the sniping members on SB anything. He said they were in contact, that the situation was resolved and encouraged any others to contact him for assistance in the future. If msscuba does not feel it was resolved to her satisfaction, I'm sure she'll share that. For some of the rest, they don't burn witches at the stake anymore either.