Offer to purchase is different from Bill of Sale.
Offer to purchase should include the VIN#, Year of vehicle and any other items included in the sale. Include a statement that the vehicle is free of any liens. It should also include a provision on how any deposit should be handled. If no contingency exits such as an inspection then the deposit will probably be non-refundable and should be stated. If an inspection is desired, financing required or insurance to be obtained then the offer to purchase should state that the deposit is refundable if the vehicle does not pass inspection satisfactory to the buyer, or the buyer is unable to obtain financing or insurance.
Also include the date upon which you will close the deal. Include enough time for the buyer to complete any inspection. It might be a good idea to put a phase into the contract with the provision that this date can be extended upon agreement of both parties.
I don't think it's necessary to over complicate this. No lawyer is needed unless some complicated issue is involved.
As a seller you should cash the deposit immediately. If the buyer has an inspection done and is unsatisfied with the inspection, he could walk and not pay for the inspection. A lien could then be placed against your vehicle.
In Florida, a bill of sale can be downloaded from the DMV web-site. The buyer then takes the completed bill of sale to the DMV along with the title and registers the vehicle. I would think most states are similar.
Tampa Bay
'07 American Tradition Cummins ISL
'14 Honda CRV