Hello,
This situation has happened to me ONCE, when I was practicing email reading, and a friend told me that she had friends on whom I could practice.
I very clearly explained to the first "friend of friend" who contacted me, that I was doing the reading in order to refine my skills, and that all I was asking in return was honest feedback, with any remark or criticism that might help me build on my skills. I made it clear that it was a deal : you want a reading, and I want experience : you get the reading, I get the detailed feedback that I need to become better at email reading.
The friend of friend accepted the deal and gave me her question.
I offered a selection of decks for her to chose from, and then I produced a detailed reading, with pictures of the spread ; it took me more several hours because I wanted it to be good.
I researched symbols, read about mythology (I was using the Mythic tarot), wrote in english which is not my first language, checked my spelling, borrowed a camera for the pics....
I really think I did a professional job, if I may say so myself.
The friend of friend then sent me a short thank you note : "thank you very much, your reading made total sense" and that was all.
I felt that her thank you note was not detailed feedback at all, and I was not happy with the deal.
So, I said nothing, but next time she was interested in a reading, I just did not answer.
It has never happened again.
Feminine-mystique, you are giving your time, and your time is precious.
My advice is, if you feel that you are being used, put your foot down now.
I understand that it is difficult to say no, but if you wait more, the risk is that your resentment willl build ; and when you eventually get fed up enough to say "no", you will be so wound up that you might say it with a lot of resentment.
Better say "no" calmly now, and explain why it is not fair that you give your time like that, than let the stress of the situation get you.
And Barefootlife is right : with all the experience you have now, you can set up an Esty shop !
Maybe people do not realise what effort and time a reading takes. If you tell them, maybe they will be ready to offer payment or feedback or anything you are willing to trade ; or they will just withdraw. Win-win !