Honestly bud this is what I hate the most, being ignorant to the new players who just want to give their two cents, but anyways that's for another topic, this is opinion on the subject:
No, having an unban option in the server shop is not useful at all. Yes, sure, it might raise money from doing so, but is it sticking the finger to the staff/players, yes. It just seems like a cash grab to have it and just screw everyone else in the end of it. Let's say perhaps, we did have something of the sort and it was like the way Pawndah describes;
Let's appose this player named "MrHackerMan" comes to the server and gets good and is actually pretty good. He starts to hack for (insert amount of time; for this example I'll use 3 months), and players are /reporting him and /helpop'ing him countlessly. However us, the staff, somehow manage to not find any evidence (Probably not true, but AGAIN, just an example), and he finally gets banned. Hooray! The players are happy, the staff can finally stop getting spammed for /reports, and everything is great. The next day, boom he buys an unban. Goes right back to using his client again cause he now knows how to keep it on the down low
If you hack and get banned, you're not going to get the easy way out and just buy your unban. That's caring about money, and not the players themselves. I don't get paid doing this, and I certainly don't want to see people that I've spent (insert time) trying to get evidence banning them, unbanned cause they have a credit card (probably from their mom, ooooo snap bb). Anyone who says "this server does it, be like them", can go to that server and stay there.
Another note: NO, FOR THE TENTH BILLION TIME, no matter how good that player was, no matter how amazing that person was to you, NO, having them back does not make a difference in player count. You're only bringing that up because they were one of your pen pals bud. I can't tell you how OG players i've seen that were unbanned not make a single difference in player count. Unless it's a youtuber with a million subs, then I guess so. I've seen at least 5 og players come back, and the only difference they did in player count was probably for one day.