this is a complicated issue, you have to get people onto the site, but you cant do that unless you have people already on the site. Without an advertising budget, you are in trouble.

I'd like to be able to search by start location or arrival location

look at meetup groups that need to go places, and allows them to rent a van or car that will pick people up at their house and still get them to their concert or ski lodge in good time. This is also a good thing for gypsy cabs; get them into the loop and have them give you a part of the fee, if you do it right they are just friends picking up other friends.