I disagree. While it is technically wrong to build an info site on a .COM or really, anything but a .INFO, that is very unrealistic of the way the web actually works and is perceived by users. Most web surfers have no idea what the extensions stand for or what they should be used for, nor do they care. Extensions seemed to be branded inside the average surfers head in the following rank...
.COM
.NET
.ORG
So, if .COM is available, regardless of the site contents, I would buy the .COM because of it's mass familiarity. let's face it, if tey remember your domain name but not the extension, they will most likely try the .COM extension first. And possibly never even try another extension if the .COM does not return the site they were looking for.
IMO, using extensions properly is not practical for the site owner seeing how the surfers do not think the same way the domain industry does.
To answer the OP, I'd go with the .NET first if no .COM is available but would seriously consider looking for a different domain that has the .COM available if your intent is to build a site to market it yourself.
Commerce site or not, .COM is most widely known and used.
Rules are OK when they are practical but when they are not, to listen to them and abide by them will always hurt you as they make you think practically within an impractical world/industry. Bad mix. It is for this reason I believe that the current domain/addressing system of the internet as we know it today could quite possibly not be the same system we will see 10 to 20 years from now, making all these great .COMs we are all buying today worth .ZIP in the near to kinda near future.