Fellow 2007 Yaris owner here - alas, RelayRides will not let me rent mine out due to its high mileage (over 100K miles). Otherwise I totally would! My renter-side RelayRides experience has been great. Obviously if you are VERY PARTICULAR about your car it's not a great idea. I'm not sure how well a Yaris would do, though - I feel like people tend to use it more for larger cars and trucks, for an Ikea trip or what have you. But maybe that's just the way *I* use it, since I already have a car.
Why not put it up on RelayRides for a couple of months and see how you do? Then if it doesn't feel worthwhile, you can sell it.