As someone who has a (very) niche product, I'm just wondering: what channels worked the best to get knowledge of your product to the right people?

I put up a test page for a niche C++ library I had developed, but haven't actually gotten around to selling it. I do keep getting a lot of e-mails from people who are interested in it though. But making sure the thing compiles on a lot of target platforms (especially Linux) is a hassle. So, what I've found, is that people need to be able to find you (duh). Try and figure out what the most likely search terms are for the problem they are solving and make sure you rank well there on Google. That'll get you 90% of the way there.

Which product are you selling?

If you wait until it's perfect you'll never sell a single copy. My advice is to launch it, get the money going, and use that to refocus your efforts on the edge platforms on which it won't compile.

People find us 100% through Google

Have you thought about allowing payment through the website? Making someone email you (a VA?) seems like an unnecessary hurdle.

We used to have that but it only accounted for a small fraction of sales. Our customers are large enterprises that have a quotation/PO invoicing cycle that pay by check or wire.

good ole mainframes still run half the computing world... we'll leave javascript to hipsters and stick to copybooks insteads :)

They run a hell of a lot more than half.