What he said.

I've used a sender that isn't quite right for my car... it mostly worked but the end points aren't quite where they should be, i.e. the tank would run dry when the gauge read 1/8 tank. Almost a great bargain, but not quite, eh? If you can get that thing perfectly adjusted, go for it.

Also, if you're checking used senders, see if you can get your hands on an analog ohmmeter; how smoothly the needle moves up and down is more important than the exact number of ohms (though it should be in that range, more or less.) You should be looking for dead spots, places where there's an open connection instead of an ohm reading. You can do it with a digital meter but an analog one will be a whole lot more straightforward as its needle will move very much like the fuel gauge does...