The ZX-81 and 1000's - working - boxed, and with good cassette software are worth more than $4.95 IMHO.
Plus you need a TV that can handle the signal, which is not always easy to find as new TV's are not set to display the 1000 / zx-81 output satisfactorily.
Anyway, once a person gets it all working properly, considering the effort they'll go through half the time to accomplish this, they'd be unwilling to part with their system for what amounts to the price of a a beer at your local bar.
Selling - Set up a system, take good pictures of the screen displaying a popular program in use, and include the memory expansion or other extras and you'll get more than $25 easy.
It's the untested units that are a crap shoot. Same goes for the Commodore Plus /4. These are another of the systems that are easy to find untested, but are rarely for sale described as "working" on Ebay.
This also emphasizes my side point that condition of the case is kind of a secondary thing, if the system does not work or is not repairable IMHO, when valuing it.
Bill