Derbydad:
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Probably because not too many people would consider putting 2 shutoffs in series.
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Actually, one of my favourite plumbing practices is to have a ball valve upstream and an inexpensive globe valve downstream on lines that aren't used often (like an outdoor sillcock or boiler drain valve. Leave the upstream ball valve wide open and close the downstream valve.
That's because on infrequently used water lines, any turbidity in the water settles out as a fine dust on the bottom of the pipe. If you open a ball valve that has dust settled to the bottom of the line, there's a good chance that dirt will get between the ball and the teflon seals and scratch those teflon seals so that the ball valve will leak after that.
By opening the downstream valve until the water runs clear, you flush out all the dirt in the line, and so your confident that the ball valve can be used. That way, you keep the upstream ball valve in good condition so that you can always repair or replace the cheap downstream valve.
I realize this situation is different cuz the upstream valve is the one that doesn't work, and this is a line to a bathroom sink that gets used frequently. But under the right circumstances, putting two valves in a row is a good idea.
Smitty Two:
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but I don't know why he couldn't do a sex change with a close or a coupling.
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How might a 3/8 inch close nipple help here?
He can't use a pipe coupling because the threads on pipe fittings and compression connections are different.
On compression fittings the male and female threads are straight, just like on a bolt or a screw or a light bulb or a bottle cap. The fluid-tight seal is made by the plastic or brass ferrule being compressed around the pipe as the compression fitting joint is tightened.
Pipe threads are tapered, and so the thread diameter actually changes over the length of the threaded area. For all sizes of pipe, the taper for NPT threaded pipe is 1 in 16. So, if a threaded nipple has 1 inch of threaded length at each end, the diameter of of the pipe threads will increase by 1/16 of an inch from the end of the nipple to the end of the threaded area. So, in pipes and pipe fittings, the fluid-tight seal is made by compressing teflon tape and/or pipe dope between the tapered male and female threads as the joint is tightened.
So, you can't use pipe fittings (that have tapered threads) on compression fittings (which have straight threads) and expect that connection not to leak.
The OP would need a coupling with straight female threads on both ends, and I don't know that such a fitting exists. If he can't find a thing like that, then he'd have to use a short piece of 3/8 plastic or copper tubing between the two valves.