How can I improve my chances of selling these books?
For a start, compare you prices with what other sellers are asking. Your asking price looks high to me, compared with other encyclopedias for sale here. I'm amazed that there's a market for sets of 40 year old encyclopedias!
You also need to change your wording. You can't say that "Yearbooks 1971-1974 will be sold separately unless a good offer is made for them". You need to explain exactly what year books you are selling, and everything you include in the description must be included in the sale.
You can't mention and invite offers for Year books that are not included. If they're to be sold separately, you need to list them separately.
