In reference to the patent: US7743760
Once a patent is expired, the claimed subject matter of the patent is considered to be in the public domain i.e. it is not owned by anybody, and anybody can use the technology without paying any assignment or licensing fee. So, in principal there is no need to buy an expired patent.
However, although the status of US Patent 7743760 indicates that it has lapsed through failure to pay the maintenance fee due by 29th June 2014 (see link to the Maintenance Fees Portal at the USPTO below), you should be aware that a patent can be reinstated by the patent owner at any time, upon provision of a statement that the delay in paying the maintenance fee was unintentional, see 35 U.S.C. 41 (c) (1) here, subject to the pre-existing intervening rights provision of 35 U.S.C. 41 (c) (2).
It is possible to purchase expired patents, and that would occur in the same manner for an unexpired patent, namely contact the patent owner (in this case the first named inventor is a DENNIS R WOODLAND, and there is no evidence of an assignment of rights being recorded) or the recorded attorney who appears to be:
David K. Benson 1712 E. Mallory St. Mesa, AZ 85203 UNITED STATES
and make them an offer.
In any event purchasing an expired patent has advantages as the owner can sue for past infringement that occurred before the patent expired (not more than 6 years from when a lawsuit is filed however).