Except for handicapped signs, no parking spot signs are legally enforceable. The store owner could ban you from returning and call the cops on you for trespassing the next time you show up though, but I doubt it.
I learned this in the Army. An MP came to teach us something about winter driving safety and mentioned that you won’t get a ticket for parking in the General’s parking space, or the company commander’s spot, or whatever. Of course, the General might have something to say about it. But it is not something that can be legally enforced.
I remember this every time I park in a Home Depot spot “for professionals only”. For one thing, I’m an engineer, which is indisputably a “profession”. For another thing, if I’m working on a project for my home, I’m saving money on labor by doing it myself. Which means I’m basically paying myself for my own labor in equity. Either way you cut it, I’m a “professional”. But it helps to know that the cops aren’t going to give me a ticket for not having a “professional tradesman” sticker on my license plate.