Used-car buyers beware -- of your sales tax bill. Starting Sunday, tax offices across Texas will use a new system to determine sales taxes on private used-car purchases, which could set the value at more than someone actually pays for the vehicle. Tarrant County is considering placing deputies or constables at tax offices when the new law takes effect. "It's going to be a little bit of a shock to some people," said Betsy Price, Tarrant County tax assessor-collector. "We might have to charge them a lot more if the vehicle is valued higher." Called "standard presumptive value," tax assessor-collectors...