I track each bike purchase and sale. 5-10% of the bikes I buy are for personal use or gifts to friends. But sometimes, bikes are bought just for the components, so it's not a perfect inventory system.

Saddles and tires often get switched around too. Also, I tend to buy consumables in bulk, like a 100 count box of brake or shifter cables, due to the many flips.

When Niagara Cycle charged for shipping, my parts orders were bigger to leverage the cost of shipping. Now, only a couple of items are needed to reach the minimum $100 for free shipping. Also, a few Amazon vendors have free shipping on less than $25 orders. So parts ordering is more as needed, not to replenish inventory.

But rather than track each dollar spent on a flip, I am considering using the standard cost accounting model, which is often used in manufacturing cost accounting. For example, each bundle of cables, housing, bearings, and other consumables is $10 per bike, rather than try to say 2 cables on this bike, 4 on that bike, etc.