besides, although the depreciation may be almost zero, it partially depends on where you plan to resell everything. If you do it on ebay you're looking at losing 6-7 percent between ebay and paypal fees, if you just used forums so you didn't need ebay you'd still be looking at 3% loss (for fees). And of course there is the time invested in buying the glass, and the time invested in selling the glass. It's somewhat risky, and you could easily lose more than you would have by just renting the glass.
Often buying the glass does make sense, if it's something you will be using frequently and you want to keep it for a long period of time. For example, if I go out and buy L glass, and then decide to sell it in 5-10 years, I will not have lost very much of the resell value, as long as I kept it in relatively good condition. Here I would be saving a great deal by buying. But if it's equipment I just want to try, or will only need for certain circumstances, then renting may quite possibly be the best idea.