Is it legal to sit in your car in front of someones house waiting for them to pull their car out so you can repossess it?

This is highly dependent on where you are at; States, counties and even cities have their own rules on what you can and cannot do to repossess a vehicle, without a court order. Check with your local City or County Sheriff's office. For example: In Corsicana Texas you can repossess a vehicle from any place, so long as you do not cause property damage, but you must FIRST notify the county sheriff's office of your intent to repossess, and show them the papers needed. By the same token, Lake Cities requires that you do NOT contact them about the repossession until the vehicle is at the storage lot, and then you must fax or send them a copy of the paperwork; all other state rules apply.

So as a general statement, yes they (you) can wait in the public street, stand on the sidewalk (even) and wait for them to make the vehicle accessible (I have followed them to the store, and ganked it while they went inside; I have taken it from their private work parking, and I have even taken one when they went inside to pay for gas (he was nice enough to leave it running, fill it up, and his date in the passengers seat - I told her to get out, she didn't - no charges filed))