A good rule of thumb for bringing cars into Virginia (and probably other states) is to have a title in hand and a completed Bill-of-Sale signed by both parties that states the transaction price. Actually this applies for in-state transactions as well.
There's nothing magical about a Bill-of-Sale. You can generate one easily on your computer. I took one from the Maryland DMV and cut-and-pasted the appropriate parts. The key thing is the transaction price and the signatures of both parties. Virginia DMV always wants to tax you on what THEY think the transaction price should be. It seems they always want to use a transaction price based on perfect condition and top market value (seemingly from a few years back!)
Gee, go figure. Virginia DMV wanting to charge you more taxes!
My personal rule is to go to DMV the very next day after the transaction and have the new title issued in my name. This prevents the previous owner from successfully obtaining a duplicate title - not that its much of a risk.
On an additional note, Virginia requires titles for trailers. Most other states don't. This makes it an extreme hassle to bring a used trailer into the state. New trailers usually come with a Certificate of Origin from the manufacturer but getting one for a used trailer is considerably more difficult. So, VA people have incentives for buying trailers already titled - not really good market competition.
Ken300D