Hello and welcome to the site.
I used to make Pysanky eggs and sell them at local fairs.
Whew, your question is a difficult one. My answer may sound a bit depressing but I will be sure to end on a positive note.
Plants are not guaranteed to create a profit even under the best growing conditions. You are limiting yourself to an indoor, non-climate-controlled environment.
You will need some kind of bank account and there may be bank fees. Postage to ship live plant material is very expensive as you will need to use Priority (fast) mail. There are times of the year that you will not be able to safely mail plants as the weather will be either too cold or too hot and the plants might arrived decayed or dead. Also, if the buyer is not satisfied you need to have in place a return/refund policy to keep the customer happy.
To legally export plant material you will need to be inspected regularly and obtain a phytosanitary certificate.
http://piorin.gov.pl/en/phytos...
So...in the end you could spend money, grow plants, offer them for sale...but if the inspector tells you that you have failed an inspection you will not be able to sell the plants.
I have an idea that is not plant-related. There are many seller on eBay and on Craftsy who design and manufacture handmade items. Do you have the ability to create something? Knit, crochet, jewelry, painting, wood carving, sculpting, etc?
If you have that extra room perhaps you could find a room-mate to share expenses; if you can find someone who is talented at making things the two of you could form a partnership and work together to make and sell things online.
Lastly, since you mention that local merchandise is very inexpensive in Poland perhaps you can offer your local merchandise for sale on the internet. I sometimes buy stuff from a few independent sellers in China and their items are very affordable.
Good luck and I hope you come up with a successful idea.
