I think a brick-and-mortar store would have the right/responsibility to ask for a valid photo ID 'cause of liability reasons, and I personally don't have a problem with being carded; I'm twenty-five and I look a lot younger than I really am. The first time at a knife shop I was carded, but after I got to know the owner (and subsequently buying a couple Benchmades from him) he knew me as a responsible individual and let me handle any knife in the display case I asked for -- personally I like to buy my knives at a brick-and-mortar store to keep 'em in business.
The only time I'd be wary of a place is if the person behind the sales counter said something outlandish like what the OP mentioned. If there isn't another more knowledgeable sales rep to speak to, then that would be an indication to take your business elsewhere...
I too would buy from a brick and mortar store, problem is there isn't a decent one around where I live. Also I had heard king of knives had a reputation for marking their prices up, and its too true. They wanted 180 for their spyderco native FRN. I understand making a profit and all, but its not like they had hundred of them sitting out the back, it was the last one they had, she even said it was their last one, and therefore on a special price.
And I'm fine being asked for ID, when buying a beer, or buying something from the local bottle shop, but I guess I just found it weird when I wasn't going to buy anything, and had told her that, merely looking. I do look a bit younger than my age, but not like I'm 12 or something.
@Karda - She probably thought I was part of a gang and was looking for a good 'stabbing knife', which is strange, as I don't fit the gang stereotype at all, and I'm not a shifty guy. I wanted to speak to the guy there, as I figured he'd mabye be better, but he was busy talking to the other customer in the store, and I only had about 10 minutes to just have a quick look.
Also about taking business elsewhere, no where else I can go really, the disposals store not too far away has a few spyderco's such as enduras and delicas, and a Tenacious or two, but they were the older models, like stainless steel endura or delica 2 (not a bad thing), and their prices were through the roof as well, 200 bucks for a delica :barf: no thanks.
Hopefully I haven't turned this into a W&C but I was more looking for your opinions/ experiences if you have had any