Any thoughts on the value of these items? I am planning to put them on eBay and, since most things there seem to do better as fixed-price listings instead of auctions (unless they're really sought-after) I want to put reasonable prices on them. (I'd be open to offers before that, though.)

• Abyss #7. This is the copy sent to Gryphon magazine for review, which means it's addressed and stamped on the back and has a "received" stamp on the front, and also a note from Dave Nalle laid in.
  
• Netherworld Continuum #6. This was published by some outfit called Cataphract Games Publishing Company, although they only took over with this issue.
  
• Vikings & Valkyrs volume 1
  
• A Challenge of Arm's (Inner City)
  
• The Ritual of the Golden Eye (Inner City)
  
• Prayer Quest (ThrowiGames)

aia wrote in Values of some old magazines and modules:I would reccomend to set up an auction for vikings & valkyrs since it is quite uncommon and it could have many interested people. Start with 10 bucks and let the auction go... my guess is that it could reach 100 bucks.
Ciaooo

Assuming enough people know about it to be interested... I thought it might be a Berg's Paradox example.

However, I had completely forgotten that I had a copy of this same volume years ago and sold it on eBay in 2011 for $176. So I guess I answered my own question on that item. :) (I have yet to see a copy of volume 2.)

I would echo some of the responses you've already received.  List these items at lower starting price and advertise them under the Classifieds.  They will find their best price.

Classifieds

You did well - congrats.

tool wrote in Values of some old magazines and modules:Prices realized, for those interested:

Abyss #7 - $88
Netherworld Continuum #6 - $41, which surprised me as there's another copy on eBay for $23 (the seller also has #4)
Vikings & Valkyrs volume 1 - $361. Now I feel really dumb for not bidding higher on volume II when I got this.
A Challenge of Arm's (Inner City) - not sold yet, not too surprising as it seems to be fairly common (although a lot of copies are missing the GM screen)
The Ritual of the Golden Eye (Inner City) - $56
Prayer Quest (ThrowiGames) - $20

Why was I willing to pay more for your copy of Netherworld Continuum #6 when there was a cheaper one available...because yours was in better shape. I don't want any copy; I want the best copy that exists. To quote my good friend Jeff, if you're not upgrading you're hoarding.
Secondly, I was the one that paid $176 for your Vikings & Valkyrs back in 2011.