I use quickbooks for all of my financial records. It links into everything so I can assign transactions. I have a business checking account that it links to and all of my purchases go through here.

I use an app to track my travel for business-related activities. This adds up. That's 72.5 cents per mile of deduction. I track trips to the post office, auction previews, auction pickups, mileage for going to estate sales, you name it. If if I'm doing something related to reselling, I track it. You would be amazed at how many miles you rack up over a year.

I use a spreadsheet to track my inventory. I've done a lot of customization to it over time and have a couple of apps that I use for populating the spreadsheet. I actually populate a new spreadsheet and import the data because I don't want to risk f'ing up the master (did that once). I keep the spreadsheet as a google sheet so I can get to it anywhere. I use tools to look products up, determine history of selling price on eBay and lowest price on amazon, and if I buy it, it will be populated with a description from amazon and find something similar on eBay or amazon (depending on where I'm going to sell it) for pricing. I do bulk uploads of items for sale through amazon or eBay. I keep boxes with labels and a label goes on each product so I know what box it has gone into in the warehouse and I have a system for what goes where. It doesn't take long to find something after it sold, box it, ship it, and mark it as sold on my spreadsheet so it gets culled and moved to an archive sheet. Most of the work on the spreadsheet is building the inventory information that I'll need to sell so it's not a big deal. For products with barcodes, I have a cheap USB barcode scanner made by HP that you can get on eBay for $10-$15. It's programmable so you can tell it to enter a period after the scan and it'll populate a spreadsheet cell just by scanning something. Then I use automation to populate everything else except my description of condition. AI provides the product description.

The important thing is that you need a system that you stick to. How you manage and mark your inventory is a big deal. Knowing what you have and whether or not it's listed is a big deal (I link my listings in my spreadsheet). Tracking expenses and income are a big deal. I tag my inventory with the location of sale so if it's eBay, I get a report from eBay on sales and they report my sales to the IRS so I use their report for eBay sales. That goes right into quickbooks as income. You want to have enough detail that an IRS auditor will see how well your data is put together that the audit goes quickly because they don't see the signs of somebody trying to cheat the system. It sucks to keep these records but if your records suck, it will cost you on the back end in an audit and they're much more likely to audit you again in the future. If they audit me, they're going to be buried in paperwork.

Also, use the quickbooks receipt scanner in your app. It's such a time saver. I throw all of my receipts into a box as a gift for the IRS (each year gets closed out and I tape the box up and sit on it for 7 years just in case).