There are several vendors, who ship systems with Debian or other distributions of GNU/Linux pre-installed. You might pay more for the privilege, but it does buy a level of peace of mind, since you can be sure that the hardware is well-supported by GNU/Linux.
Whether or not you are purchasing a system with Linux bundled, or even a used system, it is still important to check that your hardware is supported by the Linux kernel. Check if your hardware is listed in the references found above. Let your salesperson (if any) know that you're shopping for a Linux system. Support Linux-friendly hardware vendors.
Some hardware manufacturers simply won't tell us how to write drivers for their hardware. Others won't allow us access to the documentation without a non-disclosure agreement that would prevent us from releasing the Linux source code.
Since we haven't been granted access to the documentation on these devices, they simply won't work under Linux. You can help by asking the manufacturers of such hardware to release the documentation. If enough people ask, they will realize that the free software community is an important market.