Mostly just here and there. Selling some of my collectibles so I have money to buy more. I sold 40K but bought in 38K worth of collectible so my net profit is 2K before adding expense such as gas money, shipping suplies, paypal fees, etc... So in term of that, I do have to file taxes on those? And there are some item that were bought from garage sales or with cashes, how do I get proof of product cost sold if I do have to file it?
Yes, you have to file taxes on that activity.
The self employed are expected to report their income and expenses based on their own accounting records. The purpose of forms like W-2, 1099-K and 1099-Misc is to make sure the IRS knows about as much of your income as reasonably possible.
Even if you consider this a hobby, and not a business, there are tax reporting requirements.