90% of our shop used to be consignment. It's more like 30% nowadays. Tracking consignment sales are the bane of our existence. In short, it's no small task. I can only tell you how we have done it, I'm sure other people do it other ways (I'm sure some of them are better). To start... I do not really keep track of inventory. 90% of our sales are for grocery items, whose quantities fluctuate on an hourly basis, and whose units are sold by weight (not individually)... Inventory tracking??? I'm not there.
At any rate,
I created a spreadsheet where I input any new item/product... There are some crazy formulas that look at the item name, vendor ID, wholesale price and retail price, then generates a numerical 12 digit code (which I use as the SKU). Now I have my barcode for the item which contains all of the information I need to extrapolate who's item sold, and how much I owe them...At the end of the week, I'll download a transaction report from Square and paste it into a spreadsheet that then deconstructs that SKU into all the information that I need (ie. Date, Vendor Name, Item Name, Wholesale price, Retail Price, and my store's profit margin). There is another sheet which selects only the transactions for a specified vendor for a specified date range and creates a pdf which is emailed to said, vendor.
It sounds kinda simple, but really, it's more complicated than I could ever coherently explain in a few paragraphs. I've tweaked it here and there over the years making the whole process more simple and faster to use, but it's definitely still a process, with a lot of room for error. But it has saved me miles of work compared to the way we started out doing it.
You might start by assigning the item category as your store vendors... That way when you download a sales report, you can filter it down to category and you're part way there. I would definitely get away from paper, and start using spreadsheets to track data.
