Do you think the prices of the items traded on eBay determined by demand and supply or in some other way?

Yes

Another answer

I have been an eBay seller since they opened practically. I truly believe from a sellers stand point that sellers are in competition with each other within the categories but the items sell differently for each item a seller may sell. It all depends who needs what and who is looking at your items at that particular time. Anyone who is in retail sales knows about it's ups and downs.