Very important information regarding the sale of turtles. This is a very important law since many people will go out and remove the baby water turtles from the wild. They are going around the law by giving you the turtle and selling the set up

http://www.tortoise.org/general/4inch.html

In an attempt to curtail the incidence of turtle-associated Salmonella infection in children, federal regulations restricting the sale of turtles and their eggs became law in 1975. There is strong evidence that these regulations were effective in reducing the incidence of Salmonella in the US in the late 1970's. Unfortunately, although the regulations were meant as public health measures they have had impacts on both turtle keeping and on turtle conservation. The regulations limit the ability of hobbyists to buy and sell hatchlings, and, because of the expense of raising the animals, they promote the sale of wild-caught rather than captive-bred animals in commercial trade within the US.

The turtle farms that used to produce the millions of hatchling sliders for the dime-store trade now ship their hatchlings overseas (the regulations exclude the export trade) where most of them die within a couple of months. Unfortunately, released survivors of this trade have lead to red-eared slider populations becoming established on every continent except for Antarctica. These feral turtles have displaced the native species in some areas.

Whatever the merits of the regulations, the law is the law, and we have to respect this. Over the last year I have been contacted several times with regard to the legality of sales of undersized turtles. In these dealings it became obvious to me that although there are widespread misconceptions about them, few people seem to have actually read the regulations. For example: Contrary to popular belief, although they may have been inspired by consequences of the trade in hatchling red-eared sliders, the regulations cover all chelonians with a carapace less than 4 inches in length. This includes tortoises and box turtles, not just water turtles. The only exceptions are the sea turtles. These are covered by different laws. In keeping with their public health orientation, the regulations basically cover the mass marketing of turtles to the general public. Because the regulations specifically exclude sales not in connection with a business, most private party sales of surplus stock by hobbyists are unaffected by this law.

The complete regulations are reprinted below from:
21 CFR Ch. 1 (4-1-91 Edition) pages 550-552.