I’m no legal expert; I assume support can be either offered or completely avoided depending on the shop owner’s preference. Most Linux distributions come with a “this software is free (as in freedom) and comes with no warranty or guaranteed functionality” disclaimer.
If I wanted to engage more with my clients and build more trust, I might offer some degree of troubleshooting/support for the Linux machines I sold. But I don’t think I’d be under any legal obligation to offer that service just for selling the laptops.
Whether or not the computer shop offers support might affect whether or not a customer wants to shop at my store. Maybe I can sell my laptops cheaper if I don’t offer support, or maybe my laptops cost a bit more because I do offer aftermarket support.