The safest way is to ensure your buyer has an account at a local (or nationwide) branch that has a B&M location near to your house.

For example lets say I want to buy your X and I have an account at Wells Fargo. You go with me to the Wells Fargo Branch and we both sit down with a worker and you have the bank check made out to me right there on the spot and you confirm there are enough funds in the account to cover it (actually the bank shouldn't issue the check if there are not).

If your buyer is legitimate they will probably have an account at Wells Fargo, Chase, B of A, First Bank, etc... If they don't have any accounts and any B&M locations and want to give you a bank check for $35,000 from "Regional Bank of Montana" and drive off with your car the next day, tell them no thanks.