You cannot entirely avoid the risk of fraud when selling online. (I'm not sure you can entirely avoid any fraud risks when selling in person - if given cash can you be sure the notes are genuine, if meeting in person can you be sure you won't get mugged?).
I have sold motherboards. They do seem to be an item with a higher returns rate than other PC components because there are more things for the buyer to get wrong (wrong socket, incompatible RAM etc.), and more features/components that may have intermittent issues that buyers may complain about. But I don't think they are going to be a target for scams in the way iPhones are - motherboards are not really something that can be resold instantly for cash to nearly anybody.
You are right that an item worth ~£100 would need to be sent with a suitable service (Royal Mail Tracked 48 would be good), not the cheapest courier (Evri/Yodel) who won't insure any PCs or PC parts. You can set up your postage options to not allow Evri such that you will get a Royal Mail label - it will be Tracked given item value.
For your own peace of mind take photos of the item as you pack it into the postage box. Ensure it is well packed (if not in original box use an antistatic bag, then bubble wrap and a strong outer box. Fill any space with paper or bubble wrap or similar), take a photo of the box on scales to show weight. Take a photo of box with postage label on. If you can, include screenshots of the BIOS in your listing (proves motherboard can POST). None of this guarantees there won't be fraud, but you will have evidence to show eBay if required and it makes you certain that you did include the item and any accessories should a buyer claim otherwise.
