This part is a CPU board, containing a CPU (and FPU socket) plus everything needed to get it working and talking to other components, an EPROM (likely with a BIOS), maybe some RAM (though I can't zoom in on the pictures to clearly see the chips) a keyboard controller (and probably a pin header to connect a keyboard to), a speaker and what I assume is a reset button.
Boards like this would be inserted into a backplane supplying them with power and connecting them to other boards like drive controller cards, RAM cards, I/O controller cards and video cards. They're mostly found on industrial PCs.
They can not be used to create a multi processor system by sticking multiple cards on a single backplane. The 8088 isn't SMP capable.
I have some doubt the seller tested this card. Maybe he has the backplane to test it but maybe he's just hoping it works and going to refund your money in case it doesn't when a buyer tries to use it.
Edit: HanJammer beat me to it. 😁