As stated above a couple of times.
#1 cash, but I get not wanting to carry 5-10-30k in cash (what if you get setup for robbery)
#2 in person cashiers check at either bank. You know the money is withdrawn and good. Only time I won't do in person is if you feel you can trust them, like maybe users directly from this site with plenty of post history (not a first time poster, looking to buy). I feel this protects both the buyer and seller the most. The buyer has to be able to withdraw that $ at that exact moment, then the seller can deposit knowing it was a legit check. Also ALWAYS get the check with BOTH your name and buyer/seller with the "OR" statement. Just incase the deal goes south again as a buyer you can deposit that $ right back into your account. If you write the check only in their name or have "AND" then you can both get screwed. Their name only, what are you to do but recall the check and that takes time and $. With "AND" then you both have to be at the bank to deposit it in your account.
#3 wire transfer. I don't like this as much, one wrong # (account or routing) and you both lose the buy and seller. Plus it's not instant, my transaction took 3-days. I bought a MoHo this way and I was paranoid the entire time waiting for the seller to confirm receipt.
There is also the IRS issue of depositing over $9999 at one time and getting a potential flag for "income" during a audit. That's for another thread....