Perhaps, but it still makes the seller an asshole. There is something seriously wrong with the contemporary mindset that sees some sort of redeeming value in anything that generates profit - even if it would mean resorting to questionable practices and no taxes would be paid for it. This is for some odd reason starting to be regarded even as commendable with the aid of some sort of vague ideals of "cunningness" or some shit like that. The problem is, those ideals applied during an entirely different era of cultural history - now they're just mostly working against themselves. Only very few benefit from it, so why shouldn't the rest complain?
The second hand market exists because it was originally supposed to have been a very small form of business and only licensed dealers were making any real profit by selling used items. The reason why it has been kept tax free in most countries for so long is also because of the recycling aspect involved in it. That become quite big in the 80s and the 90s. Now, with ebay and the like currently, some people are exploiting something that originally operated by entirely different means - flea markets, local paper ads etc. It worked on old-fashioned notions of "honor" and "decency", which might sound absurd today to most ears of course. But if someone tried to lead you off, it was your resposibility to inform other potential buyers about such individuals. These days we're just told to "ignore them", because apparantly it's their "personal business" etc.
But you know what? The only thing we can do at the moment is to keep complaining about it. In case you haven't noticed, publicly complaining about things does have an effect. It might not be the most effective strategy imaginable, but if it slows down the unhealthy progress and potentially introduces some changes later on then I'm all for it personally. If someone is asking 1000 dollars / euros for a SH-2000 on an internet forum that person should be rightfully criticized or even ridiculed. One of the reasons why these type of ads rile up people is also the pretentiousness and the lack of any decency as far as the content of those ads go - it's perfectly normal human behaviour to react to such things and you should react to such nonsense in one way or another. When you strongly disagree with someone on a forum, you will usually voice your opinion. Yet, for some oblivious reason when money is being involved, it's just "their personal business" and you're supposed to keep quiet about it.
Bottom line: if you want to generate considerable amounts of profit by selling used items, you should become a licensed dealer and pay some fucking taxes. It's not entirely okay just because it is still currently considered legal.
So much to argue with there, but let me raise one point - and it's the same point I've mentioned again, and again, and again.
If Minimoogs still sold for $400 and Jupiter-8s for $500, would anyone
be making new analogue synthesizers? Would Tom Oberheim be selling his new SEM? Would Dave Smith bother with a Prophet'08? Would Moog Music even be in business as we understand it today? Of course not. Those high prices for used gear fuel the market for new instruments, just like $20,000 vintage Stratocasters keep the market for high quality new ones (at one fifth to one tenth the vintage price) viable.
I for one welcome high prices, since it fuels a market for less-than-stratospherically priced new instruments (which, paradoxically, can then potentially lower the price of vintage gear simply because there are newer, cheaper alternatives). I do agree that if someone makes a living buying and selling used gear they should declare it on their taxes but that goes for any hobby that turns into the basis for the bulk of someone's income. I don't care if it's vintage synths, baking cakes, knitting scarves, or carpentry.
Singling out people who try to make a profit and painting them as parasites and (your use of word) "assholes" is misguided. It has nothing to do with decency, it has to do with a buyer finding a seller and both of them agreeing to mutually acceptable terms. Who am I to criticize if someone wants to sell or pay $1,200 for a Juno-106? What if the price was $1,000? $700? $500? $200? At what point does the seller cease to be an "asshole", and who makes the determination wherein they descend into a state of assholery? Is it a binary yes/no thing, or can they be a 17% asshole for pricing 17% over an arbitrary non-asshole price?