Okay all, this is going to be kind of long, but I've seen a lot of helpful comments on here and am hoping some of y'all can point me in the right direction.

I started reselling DVDs at the beginning of the year as a way to replace my gig income (I moonlight as a pub trivia host). I have been extremely happy with the success I've had -- but lately am starting to freak when it comes to 2021 taxes. I know about the 200/$20K threshold, and will certainly exceed that. I am going to get myself a good-ass CPA who can help maximize my deductions when the time comes, but I want to have as many of my ducks in a row before I contact he or she to help me.

So here's where I'm at: I've sold about 1700 items since Jan 1. The main way I've been getting inventory are pawn shops and Facebook marketplace -- moreso the latter lately. I also have a pretty great memory -- so much so that I can look at an item sold back in February and recall where it came from and what I paid for it -- but I am nearing the end of that rope.

What I want to do is figure out what database or tools allow for importing of CSV data from eBay that I can do this once. I have all my digital receipts, but some things were paid for in cash and all others I have are non-itemized, but simple math can tell me what I paid for X or Y.

So here are my questions, in a nutshell:

1) Is there a way to add cost to sold eBay listings and then upload those as final sales, preferably with the location/mileage/shipping information attached to it?

2) If I understand correctly, only sold items count for deductions on cost, not monies paid for inventory that has not yet sold (in 2021), right?

3) I've started integrating QuickBooks, but have no experience using it so I'm sure I'm only touching the tip of the iceberg. Is this a good tool for doing what I want?

Bottom line, I know I'll need to go line-by-line for all the items I've sold, but I really only want to have to do that once. I'm also trying to get a system in place so that I can start doing things the right way for the second half of the year, and only need to worry about reverse-engineering the first half.

Any help or insight you can provide would be great. Thank you so much!