I think that marketing is one of the main advantages of selling into a very niche market - I really do no explicit marketing at all. I've spoken at various Clojure conferences, appeared on Clojure-related podcasts etc but I've never run a single ad for Cursive. Just by being active in the community (which I would do anyway, it's just part of my job) people hear about it, and it doesn't take much SEO for Cursive's website to appear when someone searches for "Clojure IDE".

Re: sales tax, it is indeed mind-bogglingly complex these days, and really requires using a provider who handles it for you. This is Stripe's biggest limitation and the single reason I'm not using them. I use Paddle, who I've mostly been very happy with. If I were really dead set on using Stripe I'd have to use it in conjunction with something like Taxamo, but that looks like a hassle I don't need and would be more expensive than what I'm paying Paddle now anyway.