My simple view would be that if the restaurant would change their system if you told them what you plan to do, it is immoral.Well, that moral principle is not going to generalize well.
But let's resist the urge to play moral philosopher and consider the facts: Would the restaurants want to stop you from doing this?
I don't think it's obvious. If I pick up half a dozen extra reservations and then try to sell them, I don't really add to the workload of the greeter, unless I pick up substantially more than end up being used. (Obviously, if you try to get 100 reservations, sell five, and return the other 95, the restaurant is going to stop you. So, don't do that!)
What you're essentially doing is allowing people to buy a better position in line. That seems perfectly rational: An upcoming reservation, like a stock option, is a commodity, so one can put a price on it, right? And it's worth noting that restaurants do this all the time [1]:
http://news.ycombinator.com/item?id=318827
...it's called "bribing the maitre'd", and they don't advertise it, even though -- if it weren't for other considerations -- it's a good thing for the restaurant. A person who is willing to pay cash money to get in to your restaurant faster is probably someone with (a) more money to spend, so they're likely to order wine and dessert and all the good stuff that pads the profit margin, or (b) a big fan of your restaurant, such that they'll pay extra to come to your restaurant instead of the less-busy restaurant down the street, or (c) about to become a big fan of your restaurant, because they can always get a seat there right away; it's just a matter of a few bucks!
So the real mystery is why restaurants don't do this themselves. I suspect that the answer is social signaling: A restaurant doesn't want to be the entity that makes the majority of its customers feel poor. For every person whom you allow to publicly buy their way to the front of the line, you send a message to an entire line-full of other people that they're just too cheap or too poor to do the same thing. The point of a restaurant is to allow people to feel rich at a fixed rate, not to force them to feel poor at variable rates. So before long your restaurant will have a reputation as "that obnoxious place which caters to snooty rich people who want to show off all their money". Unless that's your business plan, that isn't a good idea.
(It is this, not the fear of breaking some non-existent law against overtipping maitre'ds, that keeps the practice of bribing maitre'ds below the radar. It's okay to spend money for better service; it's impolite for either party to call attention to it.)
The solution that these entrepreneurs have come up with is to create two queues: The queue run by the restaurant, and a separate queue containing the reservations owned by the entrepreneurs, which they will sell for a fee. The genius of this is that there's a strict limit to how many rich people can buy their way ahead of you: The entrepreneurs only have so many spots ahead of yours. Then the entrepreneurs can fix the fee to enter their queue, which will help prevent unsightly bidding wars from breaking out in front of the restaurant. And, because the whole scheme is run by a third party, the restaurant doesn't have to adjudicate disputes, or explain all this to everyone who shows up, or spoil their reputation as scrupulously fair egalitarians who would be shocked -- shocked! -- by the offering of a bribe.
So my guess is that restaurants might be okay with this, if it's done right. (i.e. The right number of "buyable" reservations vs the usual ones, the right fixed rate, no intimidation of customers, and of course no counterfeit reservations. I believe it is the latter two problems that have given rise to laws against ticket scalping.)
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[1] How great is it that when trying to google this article the first thing I find is the HN link?