Re-entering US under Visa Waiver Program after time in Mexico [duplicate]

We arrived in the US by air, bought a car in California, and spent just under the 90 days allowed in the USA under the Visa Waiver Program before exiting overland to Mexico.

Our plans have now changed and we wish to sell the car in a few months time and return home to Australia.

To sell the car and transfer registration requires a smog check, and our preferred flight goes from Los Angeles, so it appears we have to return to the US to sell the car and fly home. We plan to give ourselves a month in the US to sell the car before flying home.

We're unsure what our best move is in gaining re-entry to the USA. Our options as we see them:

1) Return to the border after 90 days+ in Mexico, explain that our purpose of entry is to sell the car and fly out, and show proof of travel.

2) As above but take the long drive down to Guatemala or Belize in order to leave North America, before driving back up.

3) Apply for a proper visa at the embassy in Mexico City. However they appear to require original copies of marriage/birth certificates, which we don't have with us.

Additional Information:

We entered by air, and therefore weren't given a paper I-94W form. Our proof of exiting the USA within the 90 days is in the form of our entry stamps to Mexico.

We have more than sufficient funds to support ourselves in the USA, and can provide evidence of it.

We will have one way tickets booked back home to Australia and can show the booking as evidence. We're travelling on our Australian passports.

I realise that there aren't any guarantees or hard and fast rules. However we'd appreciate opinions on the best course of action based on our situation.

Many thanks!