I have an Amazon Merch account for selling my t-shirts and I also had a TeeSpring account. I could not get any traction with TeeSpring unlike my Merch account plus Merch does help with marketing and SEO on their site.
I don’t have a budget for marketing my t-shirts; so I am going the free route until the business is self supporting. After some research I have found that the best free method to advertise is via social media sites like Twitter, Instagram and Facebook. I have accounts on all of those sites and will make weekly posts of an image of the shirts along with their Amazon product link.
I have several hundred friends across all the platforms and my posts should be seen by most of them. I am also member of quite a few groups on Facebook and other sites such as Google+ and YouTube and plan to post them as well. There are several 2nd tier social media sites like Medium and Steemit that I will use that have tens of thousands of followers and naturally I will post to my blog site as well. Hopefully, I will get some traction by hitting all of these social media sites.