I wouldn't assume your mobo is dead just yet (it's not likely to be a bad mobo, and it would be a shame to buy another one only to have the same problem). as of now, it sounds like your mobo is fine; you can boot into the POST screen and it recognizes keyboard input (the beep sound comes from a speaker on the mobo) I would look at some other possible issues first...
1.
operating system - the seller says he installed Windows XP already, but it's clear from his auction listing that he doesn't know what he's talking about, or is just plain lying; so you can't go by what he said. for example, he may have skipped installing SATA drivers during the XP installation.
all your hardware probably works fine, you probably just need to format and install your operating system. he didn't give you a copy of XP, but you don't have a license for the copy of XP that's supposed to be on it anyways. if you were to swap out the mobo, unless you got the same exact one you would have to re-install the operating system anyways or it would either fail to boot, or remain unstable. either way, to stay legal and get updates/hotfixes you will need to buy an operating system for your carputer. this should be your first step. (you will need to connect a CD/DVD drive directly to the mobo to install an operating system, see #5 below for more info)
2.
power supply - if it is a hardware issue, the first component I would look at is the power supply. the seller listed the power supply as "Smart power supply w/ smart controller". that says nothing about it. do you have any specs on this unit? are you sure it's enough to power your system?
I would recommend disconnecting that "smart power supply" and hook up a standard ATX home PC power supply to test it out. if it works then you've found your problem.
4.
keyboard - are you using a PS2 keyboard or a USB keyboard? you may need to use a PS2 keyboard to enter your BIOS setup. many mobo's (including mine) will not recognize a USB keyboard or mouse until the windows drivers have loaded, which means that it doesn't know you are pressing "DEL" during bootup. this would not prevent the computer from booting up, it would simply prevent you from entering the BIOS setup menu.
5.
bootable disk - if you have the restore disks from your home PC then you already have a bootable CD to try. if you don't, then you can make a bootable CD with nero or other CD-writing programs. but since the CD drive with your carputer is USB, you probably can't boot from it. you will need to directly hook a CD/DVD drive up to the mobo using a standard ATA cable.
alternatively you could hook a floppy drive up to the mobo and boot from that as well. you can make a startup disk from your home PC by right-mouse-clicking on the "A" drive icon in "my computer" and select format, then check off the box marked "create MS-DOS startup disk" and click start. (obviously you need a floppy in the drive to do this

)
good luck
