I have done my research. In Polish market, there are basically two groups of places:
1) multibrand online stores with art and handmade stuff, where they charge high commission (up to 25%), but it is worth every cent (or grosz actually) for their marketing. They have strict entry conditions though, e.g. proper product photos are a must. Think about the way Ikea shows their product on their website - white background, no shadows. These conditions man you need to get accepted by the store management to sell there. There are people who tried several times and failed, or managed only after doing some major effort. Discussion about getting to those stores is like talking about entry conditions for Ivy League university. Doable, but not for everyone.
2) huge places selling practically everything, especially thrift stuff, often locally. Like Ebay, or its Polish counterparts. This is a good place to sell for 10$ a used polyester blanket, but for handmade goods you will receive only comments "why is it so expensive?" from people who actually want and need the said blanket. Nothing wrong with that, but such people are not my target.
Etsy seems to have a good balance, being somewhere between the two groups. It is certainly associated with handmade stuff and people who browse there have at least some consciousness about the difference in quality and price between local handmade and import from third-world sweatshops. There are no entry conditions, you can just try to sell your stuff with whatever photos/logo you have. I will make sure, of course, that my photos are good amateur level. And Etsy will not be the only place for my business, but I chose it for the first one.
When I decided to turn my hobby into a source of modest income, the first rule I adopted was: (almost) zero investment before income. Which means: I only invest what I have earned with this selling, I do not spend my savings on it. E.g. I sell a quilt or two, maybe some pillow covers etc. - I buy fabric for next product and save the rest towards the cost of e.g. the aforementioned product photos. After some time, I hope to get accepted at "Ivy League" multibrand stores from point 1.
To be exact, the zero investment is only possible, because I had already made "investment" in quilting as hobby. I do have my machine, fabric stash, threads etc. But the cost of this cannot be actually called "investment" because whatever is spent on hobby, is not expected to pay back/pay off, which is the definition of investing.
Thank you all for all the tips and for reading so far. And further tips are welcome!