[homogeneous]

I just don't see how ethereum (or even bitcoin) is useful here. If you make a deposit on a rental vehicle then the deposit will be returned when you return the vehicle as it was given to you. Why is a smart contract needed?

[jsprogrammer]

When you do not want to be there to physically collect the deposit?

[homogeneous]

Certainly a smart contract is not necessary for that.

[jsprogrammer]

Can you link to such an implementation?

[homogeneous]

I'm not really sure what you're asking for. If you're looking for a system capable of accepting payment without an attendant there are many. Here is just one example:

http://bw-2e2c4bf7ceaa4712a72dd5ee136dc9a8-bwcore.s3.amazona...

If this is not the type of device you're talking about please be more specific.

[jsprogrammer]

I was thinking of a system that would let you place a deposit immediately before rental (imagine a rental car in a public lot) and then automatically receive your deposit back when you are done with the car (which was possibly even left in a different lot). I'm thinking the car would use its sensor system to detect if any damage might have occurred and if not, it could trigger the return of a deposit as soon as the vehicle is locked.

[homogeneous]

The system you describe is already possible today without the need for a smart contract.