Close to zero unless it's fancy stuff and you do a lot of work. I'm involved in a family move and we gave most of it away.

A lot of the stuff you're mentioning is basically worthless. Given that he has time he can get prices probably around ebay or goodwill value.

Working CRTs can sometimes go for a bit because of retro gamers.

The more common companies for reselling is for the company to charge you for removal because they will take unresellable junk as well. They are usually the option when selling/donation is not and disposal is more of a pain otherwise. I had to get rid of damaged couches and other furniture recently and to get a dumpster would have been like 500 and then id have to get it in there myself whereas the company charged 600 and they removed it without my help.

Mattresses are usually not resellable and it might even be illegal to do so because of regulations to limit bedbugs.

Music players probably obsolete 0-50. Cutlery set under whatever Costco charges for a set so like 10 unless they are actually silver or something. China and coffee maybe 10-20 per set. China may not sell as younger gen doesnt really use. Lamps 10-20. Books magazines and clothing selling by box/bag you can probably do 10-20 per.

Decent furniture can go for a bit even if the buyer has to transport but 10-20% of original value seems about right.

Things like the 40 year old computer monitors have a negative value, unless they're something special like a 21" Trinitron or an IBM T220/221.

Unless the furniture and furnishings are of a sought-after style (e.g. Art Deco), they're basically worthless.

I sold most of my belongings back in 2009. Furniture and beds had basically zero value. A solid wood table retained decent value. Electronics were worthless unless they were vintage collectable (e.g. Rotel amplifier). eBay was the best place to sell things because people are committed. Craigslist and Facebook Marketplace are a huge time-suck because you get so many flakes, low-ballers, and tire-kickers.

There are companies that specialize in running estate sales. Either due to death or moving into assisted-living. I'd probably start there.

My friend's a dealer that does stuff like this. Basically her and a friend will go through the house, pricing everything. My friend either knows the price of something offhand, can look it up, or, worse case, will consult with another dealer specializing in that item. It can be tedious and time consuming.

Then they'll offer the merchandise to other dealers that can come in and scoop up a bunch of merchandise, or my friend will buy it herself to re-sell. If something is really valuable, it might get sent to an auction house.

After that's done, it's time for a yard sale, the remnants of which go to a thriftstore or a landfill.

I don't know what percentages are but don´t just go with the lowest bidder. The ability of people to absolutely fuck you over here is EXTREMELY high, so you want to have people with sterling reputations over people who are low-balling you.

Go to antique / thrift stores / consignment shops in your area and ask around for dealers who can do this.