It does depend on your how you plan your display....I've seen minimal jewellery displays using blocks of styrofoam and placemats, mini pillows or mini frames.
There are some shows where you can rent tables, and some rare shows that provide a large tent, with multiple vendors inside.
Do you know anyone that you can borrow a car from? I have a friend who rents a car when she needs one, but it is an added cost. There are also some car sharing services with more reasonable rates than a full rental - Boston may have one, if you do some searching.
The only rolling cart that I can imagine being allowed on public transit, is a handtruck dolly, where everything stays vertical. I can't imagine a cart dolly being allowed. Use bungee cords or straps to secure the load.