It's good that you are admitting 6 people didn't receive their skulls or money back and I understand life is hard, I'm not going to make this about me, but believe me, I understand, making props is my full time job and it has been very hard for me this year.
However, you just can't let people take a, what was it, 600-800€ loss because of issues between you and your partner. I understand things are hard, but you need to take full responsibility for this, you have a job, you admitted you won't be able to deliver those skulls, ok. Now, every month, you pay back 100€ to each people on the list one by one until every one has his money back. You need to consider this as a loan, people loaned you money for your business a few years back and now you need to pay your "investors" back over time.
Cheers