Who owns what? Like other extinct publications, it's difficult to tell who owns what, or if elements of the old magazine are even 'ownable.' If you're betting the odds, then Amazing Stories covers from 1923 through 1963 are most likely in the public domain. That's because these works must have been renewed and most works were not. That's not a definite, but it's a strong possibility. (BTW, you can hear an interview on the PD with author Steve Fishman, here.)
Why can they sell them? You'll notice that the illustration we are featuring includes a copyright notice from this company. As you can see by following the link, the company sells high quality digital versions of Amazing Stories covers and other pulp magazines. How can they do this? One possibility, though unlikely, is that this company acquired all the rights and they now own the copyrights hence the placement of their copyright notice. (By the way it's illegal to place false copyright notice information on a work though it's a common practice). Another possibility is that the company researched the Copyright Office records and determined that the work was public domain. (If that's the case, placing a copyright notice on the work is improper because anybody can copy it freely.) A third possibility is that the work is 'orphaned' -- it's still protected but nobody is looking out for it. And that's why this company can risk offering it.
What about trademarks? It's likely that the last buyer of the Amazing Stories catalog acquired trademark rights. However, nobody appears to claim a valid federal trademark registration to Amazing Stories. In any case, we wouldn't worry too much about that particular issue unless you begin making or selling new products with the Amazing Stories name or logo, or unless you claim some affiliation or association with the Amazing Stories brand.
Your course of action. You can double-check Copyright Office records to see if you can find any renewals for covers prior to 1963. If there are none, you are free to sell your restorations of covers from that time period. You shouldn't include a copyright notice because restoring a public domain work, as difficult as it may be, does not amount to copyrightable subject matter. That doesn't mean you can't be proprietary -- for example, by installing a visible watermark as a Photoshop layer.