States collect sales tax when the new owner applies for registration of the vehicle (this is essentially how you transfer ownership). It may not be so straightforward to just transfer a title from one party to another unrelated party without a hassle; essentially they'll assume a sale was made, money transferred hands, and, depending on the state, they may assess a value for the sale, despite your claims.
You need to try to find out what the rules are within the state you expect to do this.
Also, even if you don't pay sales tax, you may be paying other taxes (luxury tax, excise tax, property tax, whatever). In Washington state, I pay $24.00 a year for RTA tax (that's for mass transit). It cost me $67.75 for registration of my F650 last year which included the RTA tax - you may pay similar or more just to transfer a title. Also, depending on the location within a state, there may be additional taxes allowed the local county (King County, where Seattle is located, was allowed to access a tax of over $100 a year for vehicles registered to owners living in the county - for a monorail project that has since been cancelled).
Chiang Mai, Thailand