Anywhere you feel safe handling (usually) a reasonably large amount of cash and an item of equally high value with a complete stranger... It probably makes sense to consider locations in which the presence of a firearm is not totally out of place - a large sporting goods store parking lot is a good bet.
If you are uncomfortable with a straight-out person to person parking-lot sale I suggest working with a local FFL, let them know you would like to transfer a firearm to another private party and pay the appropriate fee (usually around $25) for the service. Going through an FFL is, in my opinion, well worth the fee for the peace of mind provided the buyer and seller; they usually run the SN to verify a gun isn't reported stolen which is good for the buyer, while verifying buyer eligibility via the background check for the seller. Working with an FFL to facilitate the transfer also provides a safe place to meet and perform a more thorough inspection than is possible while remaining inconspicuous in, for example, the Cabelas or Fleet Farm parking lot.
