I think this is more of a thinly disguised rant about a specific business than a serious question about business etiquette. But I'm going to pretend for a moment that it was something to do with the thread title... and then I'm going to turn it on its head.
While I'm sure you're right and some people could certainly do with business or etiquette lessons, I think a lot of the problems stem from the fact that a lot of customers have to learn not to be assholes too.
I had so many complete fucking morons who expected me to jump through hoops for the buck or two they spent that I all but gave up selling stuff. Too much hassle.
So how about people learn some customer etiquette and how to deal with 'businesses' - and remembering that an SL business is usually one person designing/creating/manufacturing/marketing/supporting an entire product range, not some faceless multinational fronted by some grinning droid who's paid to field your complaints.
Here's a few thoughts:
2 measly bucks does not entitle you to lifetime fucking support 24/7.
2 measly bucks does not give you the right to get rude when I don't answer your IM in the first 10 seconds.
Why is it that people think that when they IM they're the only person in the world I'm talking to or that I'm able to drop everything to reply? Hell, I usually have so many IM windows open that they drop off the end of the window and I don't even SEE new ones. So if I log without answering you, its probably because I didn't even know you IM'd, not because I'm rude or don't care.
Or I might really be in a rush - like, too much of a rush to IM every one of the people who bombarded me with IM's when I logged in and say "sorry, I can't stay - I'll deal with you later". basically, don't assume you know my situation and that I'm just doing it to be rude or something! You don't know shit. Hell, I coulda just crashed hard. Again. So following up your IM with a post-log one saying "how incredibly rude to log while I'm talking to you!" is guaranteed to piss me off even more and exponentially lower your chances of getting any help...
Just because I create content in SL for sale does not equal my being at your beck and call. SL can still be a hobby even if you run a 'business'. And anyone who doesn't get that a business owner isn't going to be logged in right there and then to help you needs to get a grip on reality. And stop buying stuff, probably. I just create stuff, ya know? If you wanna buy it, good for you. But I didn't just enter into a life contract with you.
So then when you do answer your IM you find out that Mr Angry is only angry because he can't use the product because he couldn't be fucking bothered to read the carefully written manual first.
Like, "this is a rip off it doesnt work I want my money back" isn't the best way to start the conversation when it turns out you can't get it to work because you're fucking stupid and didnt read the instructions to learn the command to turn it on. It just guarantees that you're not going to get an awful lot of help from me.
2 measly bucks does not buy you hours of my time answering basic shit like how to attach it or recolor it or resize it or... if you can't use SL, thats not my fucking problem. What am I, Live Help?
2 measly bucks does not make you my friend, and no we can't exchange friendship so you can stalk me when I'm busy not answering your IM's

2 measly bucks does not give you the right to demand a refund some 6 months later because there's a better one on the market now and you 'wasted your money' on my product (this happened! more than once!)
Even better, people want refunds for no transfer objects! "Oh, well, I've deleted it and emptied trash. I really don't own it any more. So you'll have no problem refunding me?".
2 measly bucks does not give you the right to pump me for information on how it works so you can clone it/use the script/rip me off.
I could go on for hours about the morons I've dealt with. But I won't.
I'll just summarise by saying that if you are an asshole to me I'll be one right back. And I'm actually probably better at it than you, too.
Then I guess you can come present your story in the forums and tell the world what a horrible person I am.
Except they know already

Ah, I do love a good rant on a Monday morning.