You're dealing with a kinda unique situation where properties flip from inventory to sales fast. So, to manage this smoothly in QuickBooks Online, you gotta have solid tracking for the purchase price, renovation, and closing costs. Here's what you can do:

First, set up each property as an inventory item; this treats it like stock, showing up on balance sheets and letting you record costs bang-on. For tracking the different costs, maybe set up accounts for each type: purchase, renovation, and closing. This lets you tag expenses straight to each property.

For recording these costs, you can use the "Projects" feature if you have QuickBooks Online Plus or Advanced. This helps you tie all costs an incomes back to specific properties, showing clear profit once one sells.

When recording a property's sale, you'll wanna match its cost with the sale price. Do this by adjusting inventory, which moves the cost to COGS, making sure your profit margins line up right.

Challenges pop up if your cost allocations are complex or change alot. If that's the situation, a deeper integration or a third party app might be worth looking at to automate these allocations more.

Managing can be tricky without a good grip of QuickBooks features. If you're not comfy setting it all up, working with a QuickBooks certified accountant could save you headaches and make sure everything's done right.