Router planes are a bit of an oddity among tools because Paul Sellers sung their praises. Since then, prices have been high for good examples, and even less than perfect ones are priced by the sellers accordingly. The price of Stanley and Record bench planes also went up noticably after he recommended them as the most cost-effective way to obtain a decent plane. Sellers also mentioned a while ago that he regarded Groves saws as among the best he'd used; their prices shot up, too. (Groves saws are good, but they're not three times better than a whole raft of other Sheffield makers.)
I suspect that this reflects the nature of the vintage hand tool market; supply is finite, and demand has increased a bit since the advent of the internet hand-tool guru.
It's also an inevitability that vintage tools will, for the most part, have won a few battle scars over the decades - and that's not the seller's fault, they're just offering what there is, in an 'as is' condition. I found when going through a saw-buying phase a few years ago that about 50% were duds (blades not quite straight enough, usually), but I took the view that as I was only paying about £10 a pop including postage, assembling a set of working vintage saws for the equivalent of £20 each was still a lot more cost-effective than buying new ones. Had I been willing to pay more for each purchase, the dud rate would have dropped. Yer pays yer money ...
I think you have to approach Ebay with a slightly fatalistic attitude. Over the course of a couple of years, you'll snag a few absolute bargains, and you'll also be lumbered with a few pigs in pokes. Most acquisitions will require at least a clean and sharpen, and possibly a lot more tlc to give a good working tool. If you want something that needs minimal work then buying from a dealer who knows what they're selling, or looking for pristine examples and being prepared to pay the higher price, may be a better approach.
It's a disappointment to snag a dud. However, if the seller has provided a good set of photos, it's up to the buyer to look and decide whether the risk is worth it. Bear in mind that some sellers may not know a router from a rebate plane, just offer what they have in their hand in good faith. The risk can be cut by researching sellers and only buying from the ones who clearly know what they're selling and give honest and full descriptions - but you'll have to pay the premium.
Your choice. If you want risk-free buying, stick to new stuff from reputable retailers. Anything vintage is a bit of a gamble.