FCC law does allow for building personal homebrew transmitters. This law is pretty clear that your homebrew is not to be a kit (see below).

The idea behind operating and building without certification is that you are supposed to have the engineering background to be able to homebrew (design from scratch) a transmitter. This means that you probably have the knowledge to insure it isn't interfering with other services. Also there is a limit of (5) operating homebrew transmitters. As the builder and operator of the homebrew device you are responsible for any undesired emissions it may produce. The rules state that good engineering practices should be maintained to prevent such undesired emissions. Here is the FCC rule, Equipment authorization means certification.

If you do build a kit, you are responsible for any problems or interference it may cause, not the kit maker, even if you have no technical ability.

See this question posed to the FCC about kits and their response:

Inquiry:

We would like to inquire about the legality of operating a Kit intended for Part 15 use. These kits are prevalent, for both the FM and AM band. Most of them advertise they are Part 15 compliant. Here are applicable rules we have found:

Part 15.3(p)
Kit. Any number of electronic parts, usually provided with a schematic diagram or printed circuit board, which, when assembled in accordance with instructions, results in a device subject to the regulations in this Part, even if additional parts of any type are required to complete assembly.

Part 15.23(a)
Equipment authorization is not required for devices that are not marketed, are not constructed from a kit, and are built in quantities of five or less for personal use. We would appreciate direction in this matter, Thank-You.

Responce:

The kit does not require equipment authorization to be marketed (sold) but they would still need to comply with the limits set forth in corresponding sections of part 15. The responsibility of compliance lies on the user, or the person who assembles the parts together, not on the manufacturer.

So it seems clear that the builder of the kit assumes all responsibility. There is no requirement of compliance on the manufacturer.