Freelance, Hobby, & Spring Cleaning Questions

Hello and thanks for hosting this event! 

I'd like to know more about the $600 limit for self employment or side hustles and what it means for me, as some of the information I've found isn't the most clear. 

I have a full time job, employed in Arizona. In the past I've picked up odd jobs - house/pet sitting, graphic design, and selling old collectables or household items I no longer need. I've hesitated with any side hustles lately because I don't want to report incorrectly and be audited, and don't want to put in extra effort if any small amount of income I'd make is taxed based on self employment and I only break even. A few different scenarios below that I'm curious about. 

1) I have a hobby where I purchase collectables for myself. Sometimes I buy one and decide I don't like it - whether I just opened the box and dislike it, or a year or five later when I no longer want it in my collection. Sometimes I don't have the receipt available to prove what I purchased it for - online is easy enough to show, but often I'll purchase in person from other collectors and it's not the sort of thing where you make a reciept (like old school trading Pokemon cards). How do I show I'm not making money on this sale (possibly even at a loss with shipping factored in), or not making much? It's a hobby not a business, and sometimes I just sell the collectables I no longer want in my collection. 

2) Similar to the above, but in the past I've sold old household things online using Mercari. Shoes or an outfit that came in the mail and didn't fit me, a bracelet I no longer like, etc - just small things to tidy up my home and that are still good enough for someone else to use. 

3) Within the aforementioned collectable hobby, people create and sell artwork based on the collectables. I've thought about doing this, but needing to report on my taxes that I'm self employed makes it all more confusing and although I want to sell my artwork, I don't want to make a mess of my taxes, as I do them myself using TurboTax. I don't think I'd be making more than $600, but a single collectable artwork could sell from $20-300, so there's the possibility to go over $600 if I sell a lot. Even so, I'm not a small business (yet), but all of my art supplies have cost a lot of money over the last few years. And buying/producing doesn't all happen within the same year - I've bought a lot of art supplies in the last four years and I'm just now starting to produce artwork for myself, and to gift to friends/family. I'm just not sure how to go about this; if I did decide to sell my artwork in the future, I don't understand how have it be straight and clear with taxes and still make a bit of money for the work I've put in. 

4) Furthermore, in the past I've done graphic design on the side. It's never been more than $600, but what tax reporting do I need to do if it's less/more than $600? 

5) What about any combination of hobby/artwork/graphic design/other income? Whether selling my hobby collectables, selling my artwork, providing graphic design services, or selling off old household items I no longer need - how does the combination of these things add up to impact my taxes? I want to ensure my own financial/tax safety and understand but it all seems so confusing, especially for someone who hasn't run a business and does their own taxes. Thank you very much for the help and insight - I appreciate this opportunity!