Whenever I post something up for sale I always do a search to look for prices on similar items. I don't want to be over-priced but I also don't want to give stuff away.
I also search and look at multiple items for sale before I buy. Past sale prices are relevant here as well.
I simply can't fathom why someone thinks this info is irrelevant. The market fluctuates.
I don't think it is as important to have as having sold shit marked sold however. Nothing more irritating than finding something for sale and trying to buy it and the seller ignores you because they didn't have enough concern to mark it sold and now they are tired of people asking if it is still for sale....