many items are not illegal to make, sell, or possess.
but they are illegal to carry on your person.
the question becomes how do you transport an item illegal to carry, but legal to own, once you have purchased it.
if its in a bag, with the receipt, that's one thing. if its in your pocket, that is another.
if you walked to the swap meet, threw away the receipt, and didnt ask for a bag, you may have created your own situation.
dont expect any officer or district or city attorney to know anything about knives. there are many many codes, sections, and laws. most will have at least a familiarity with weapons sections, but when talking about the specific function of a specific knife, it wouldnt be fair to expect anyone else to know that.
im a knife nut, and a cop. i know about many knives and their functions as well as the applicable sections within the areas i work. but i dont pretend to know everything about every knife and whether it could be a violation. i may have an opinion, but it is only that. some knives are clearly switchblades or autos, and easily fit the definition. but some are not so clear.
is an assisted opener a switchblade? i dont think so, there is an exemption which i believe currently applies. but you could ask a hundred officers, da's, ca's, and judges, and i might be the only one that thinks the way i do.