The main thing to check whenever purchasing a second hand sinclair computer is whether it has been tested. If someone lists it and says that "it was working when put away years ago" you have to wonder why they have not bothered to test it before listing - is it because it doesn't work at all anymore, or maybe it is just the keyboard which no longer works.
If the keyboard no longer works, it is normally quite easy to replace a keyboard membrane on the Spectrum and Sinclair QL - replacement keyboard membranes for the ZX80 and ZX81 are nigh impossible to find, and even if you come across one, it is likely to be 20+years old.
Often, the seller will say "starts up to F1/F2 screen" - this is the boot screen. If they cannot get past this, you can bet your bottom dollar that it needs a new keyboard membrane (cost about £20 plus p&p). They may also not have tested the microdrives - check that they can load software and save to both mdv1_ and mdv2_ as often the microdrives are worn. Sometimes all it needs is someone to push the small rubber roller back down its shaft to get the microdrives working again, but you never can tell.
Remember a QL is never rare, unless it is boxed and in A1 condition - there were about 100,000 made and less than that ever sold in the first place.
Again, if a seller only lists that they can get a TV picture but can't remember how to load a game, it is probably that the keyboard (or some keys) are not working. A replacement keyboard membrane for an original rubber key spectrum will set you back about £10 including post and packing, although you will often damage the metal faceplate around the keys when replacing this if you are not careful. A replacement keyboard membrane for the Spectrum + or Spectrum +128K will set you back about £20 and is worth more than the actual machine (if it is a 48K spectrum).
A ZX Spectrum + 128K (without the in-built tape deck) and a 16K ZX Spectrum are the only rare machines. The others were sold in their millions!!
Replacement Membranes - A Warning
The replacement keyboard membranes we sell are brand new (manufactured in the past couple of years). You sometimes see some fo the old stock lying about - however for Sinclair QL membranes, the last manufacture run was over 10 years ago now, and Spectrums about the same. That is why we have to charge more, as our membranes were made at today's prices !! As a membrane ages, it deteriorates due to ultra-violet light - modern ones will last longer, but the expected life span on original membranes (even unused) is no more than 10 years.