I honestly do next to no marketing for my SW shop and I get a relatively steady influx of customers every month. If I had to guess, I would say most of my customers are people looking for specialized products that wouldn't sell well enough for a larger company to make a profit (making hundreds of thousands of a given thing through traditional manufacturing methods has a startup cost that is prohibitive for smaller runs, under 1,000 pcs, for example) but are desirable enough that people are bound to find my stuff through searching the Internet for similar products. Remember, Shapeways does their own marketing for their site so searching on Google using keywords related to your products may very well take them directly to your shop or a page of search results on the Shapeways site that shows your shop or shop items.
To dive further in to my own experiences - as a base of comparison, my main offerings on here are miniature skyscrapers and video game replicas. I don't target specific audiences, per say, but I know there are a lot of skyscraper nerds out there (myself among them) who are looking for specific models in specific scales. The beauty of 3d printing is that I can resize any model to print in a different scale and I actually have several repeat customers who browse my selection of skyscrapers every few weeks and ask me to upload models I've already built in the scale that they collect. It's a minimal amount of work for me to put in for a guaranteed sale!
As for the video game miniatures, I get a lot of action figure enthusiasts buying them because I create items to scale with popular figures and the majority of the items I create are not something a larger company would include with a given figure or offer on their own. And much like the skyscrapers, I have customers asking me for different scales of models I've already uploaded or to create items that aren't yet in my shop. However, I know for a fact that most of my customers in this area find me through Instagram where I post a lot of images of my 3d models so that's a good place to start if you want free, effective marketing. If you already have an IG following, all the better!
As a general rule of thumb, your shop will do best if you a) create kickass models and b) offer things that are popular enough that people want them but not popular enough that larger companies are offering them for a fraction of the price to 3d print them.
Of course, it also doesn't hurt to do some of your own marketing, but I'm proof that you don't necessarily need to in order to make sales. One thing I will say, however, is that you want a lot of models uploaded to showcase your ability. If you only offer a dozen products, it isn't likely to draw a lot of customers (the less you have, the less you sell) so get a good amount of offerings into your shop to attract more customers, provide more options and to present a substantial shop that will be worth revisiting. As of now, I have just under 600 products across 9 sections but I've been at this since 2012 so if you're just starting out, maybe come up with a line of products that are creative and original in their inception but don't take hundreds of hours to create. Then branch out into more complex and unique models that are real eye catchers! Also, find a category of products that aren't already plentiful on Shapeways. iPhone cases, for example, are super popular but that market is saturated AF and your products will likely get lost in the shuffle unless you're offering something truly unique.