Lets say you need a decent telephoto lens for action shots, but you don’t need it all the time, and there are places that charges a rate per day to rent them (typically about 1000 a day for a decent Canon L series lens). Would it be cheaper if you buy the lens used on ebay or something, use it for the amount of times needed, then re-sell them?
You can typically sell Canon’s L lenses for about 80-90% of what they are new. Buying used doesn’t save you much, but then, you can sell them for about the same so you don’t usually lose too much.
If you’re going to use it from time to time, it’d be better to buy and keep or rent. The only way buy then sell would work is if it’s for a specific shoot. Even then, renting would be much easier.