I can't comment specifically, because I don't know what sites, but I'm happy to review if you give me the names and/or links.
Appraisal is based on a willing seller/willing buyer model. I've found records of quite a few of these that have sold over the past few years, none over $400. (Keep in mind that appraisals are based on prices ACHIEVED, not prices ASKED--particularly eBay asking prices which are often amateur and unrealistic!)
In general, vintage electronics, especially the larger cabinet pieces, do not sell at high prices. Few collectors have the space for them, and shipping and transportation are an additional problem. I've never seen a vintage stereo of any kind sell for the prices you are mentioning (and I've been a full-time appraiser for 22 years), so I'd love to see those links.
There are some examples currently available, with slightly varying cabinets and years, on sites like Chairish and Etsy, all for under $400. While the original box and manual add a small amount of value, they don't add a significant amount.