The most popular cassette player is the Fisher Price Tuff Stuff with Voice Warp. I consistently sell this on Amazon for an average of $69.99; last Christmas they were selling as high as $89.99.
If you find one, make sure you test the microphone to be sure the voice warp works; it seems to be the first thing to malfunction on these players.
Fisher Price Tuff Stuff Tape Player with Voice Warp
Fisher Price also made a standard Tuff Stuff cassette player. I've sold this one for $49.99 on average, and I was lucky enough to find on new in the package that I sold for $110.00.
Standard Fisher Price Tuff Stuff Tape Player
Other kids tape players/recorders sell for a nice profit. Two weeks ago I found this purple Playskool Tape Player for $5. Sold in 1 week for $29.99. I've sold this one before so I just keep reusing the same listing. Very quick and easy flip!
There are a number of reasons that these still sell even though compact discs replaced cassettes. I've sold these to teachers who have a lot of books on cassette. They are not as delicate as a CD player, so they are great for schools and daycare centers. I've also sold them to parents of special needs children, as they are easy to use. Finally, adults with poor eyesight can see the buttons to listen to music or books without assistance.
There are lot of different makes and models of children's cassette players. I stick with the brand names like Fisher Price and Playskool. I've always kept my prices high and have never had to wait long for one to sell. I've seen some people price them low on eBay, and that's ok because I just buy them and put them on Amazon and make a nice profit!