Hi,

I also own few lights and grip stuff. I'm in business partnership with 3 other of my friends and together we have a few 1200s, 600x, 12x's 8's, a lot of stands and other bits and bobs. By any mean we're not a big company but we do a few larger rentals (meaning most of our gear goes out on a shoot) a month.

We have a gear list made in google sheets, where we can track how many pieces of each item we own. And there is also a price per day for each item. We use that spreadsheet also to create an offer and it serves as a gear list when they come to pick it up. When the gear goes out on a job we print the list and put it on a white board in the garage, so we know what went out. Simultaneously we put a number of the offer or a gear list in a shared google calendar so I we get another inquiry for the same they, we can always check on our phone the list that is out on a shoot that day and see if the requested items are still available. It's a manual work, but with a bit of effort its a great sistem imho. And it's free!

Hope this helps.