Gun rights advocates argue that policy aimed at the supply side of the firearms market is based on limited research. One consideration is that 60–70% of firearms sales in the U.S. are transacted through federally licensed firearm dealers, with the remainder taking place in the "secondary market", in which previously owned firearms are transferred by non-dealers. Access to secondary markets is generally less convenient to purchasers, and involves such risks as the possibility of the gun having been used previously in a crime. Unlicensed private sellers were permitted by law to sell privately owned guns at gun shows or at private locations in 24 states as of 1998. Regulations that limit the number of handgun sales in the primary, regulated market to one handgun a month per customer have been shown to be effective at reducing illegal gun trafficking by reducing the supply into the secondary market. Taxes on firearm purchases are another means for government to influence the primary market.