Each receipt is entered in detail thus giving me an accurate count.
All sales are entered as received from the various market places.
A year end inventory adjustment gives me my COS -- accurately.
If I ran this full time and/or it was my main source of income I see the value of having up-to-date sales, costs and inventory info but right now I just settle for accurately reporting a P&L at EoY. What you do by entering details in IL and just moving the bulk numbers into Quickbooks is something I may look into. I assume IL has a tracking account for sales tax incurred.
My way doesn't give me monthly valuations or profitability but I can determine individual profitability pretty quickly when I determine whether or not to send inventory in. Generally I use the 30% rule to determine. Take 30% off the selling price and if the remainder doubles my buy-in I list. I just make exceptions as I go. i.e. categories like HP or SC I may hold a bit longer because they tend to continue to appreciate while other categories I may dump early taking that initial pop knowing there are diminishing returns after that. Sometimes just clearing inventory space of random items in the warehouse is a motivating factor.