I have never, not once in my entire life, had a problem with DMs being "unfair" because of the shop system.
considering the rest of the post is based on your own personal experience there isn't much i can say since it doesn't change how you've played DM's in the past;
but i'll tell you that for me it has always been the opposite. i have never joined a server, or even played a game for that matter, with a shop system in multiplayer that kept itself well balanced. new players have always been inferior to the older players from my standpoint.
i completely agree with racerboy that a shop system is bad if the difference between the first weapon you have and the last weapon you can get is super overpowered; the problem there being that if there isn't that difference, the shop system basically becomes useless, because then there's no incentive to getting the overpowered gun.