Does code prohibit using salvaged materials in new electrical work? Can wire, receptacles, switches, wirenuts, boxes, connectors, etc... be reused?
As long as the components meet current code they can be reused. For example if I replace all the switches and outlets in a home and the outlets are 3 prong and in good shape I will save them and the switches for small repair jobs. You would think after 35 years of doing this I would have truck loads but I only save the quality parts; backstab only devices usually go in the trash and I use enough for friends etc that I only have a medium box full.
Wire can also be reused but I usually will not save romex under 10 awg and then if only in like new shape. I will regularly save larger thhn/thwn wire and inspect it prior to reusing.
Even breaker panels can be reused if they meet current code but in many cases I save the older quality panels for subs in a pump house or garage because most of the time they are only 60 to 125 amp. With all the new AFCI and GFCI requirements I prefer getting larger panels for a home.
"In good and proper working condition."
K601.4 Material, equipment and appliance reuse.
Materials, equipment, appliances and devices shall not be reused unless such elements have been reconditioned, tested and placed in good and proper working condition and approved.
K601.3 Alternative materials, methods, equipment and appliances.
The provisions of this code are not intended to prevent the installation of any material or to prohibit any method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An alternative material or method of construction shall be approved where the code official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety.
To the best of my knowledge, no one's code forbids the reuse of all most anything (that the manufacturer does not explicitly state 'single use only' - like flexible gas lines), provided it is in good working order. For which you have to use common sense, but ultimately it's up to your inspector's discretion.
So, that box of corroded EMT fittings from 20y ago rotting in your basement: throw them away. Non-TP receptacles; 99% of the time you can't use those anymore either. Don't reuse wire nuts without making sure there's no broken off wire in there, and that the insert isn't mangled. Used toggle switches with a weakened spring are garbage. Used wire is fine as long as it's (all together now, class :) in good working order.
This is a question you don't see very often.
In answer to your question. I do not know of any part of the NEC where it specifically states "you can't reuse electrical material, devices, light fixtures, or equipment".
However, that does not release you from other rules and laws. I think in terms of a contractor and if you are working under a contract, it usually requires you to use all new material. Also we really don't like being sued, when a customer is assuming he is getting new product and finds out it was used.
Now have I installed used product in the past? Absolutely, most of the time I did it under the "brother-in-law" job or certain charity work. If you think about it some equipment is obsolete and the parts are no longer available like a Zinsco breaker. There is an entire industry that buys and sells used equipment like this.
I will conclude by saying that when I did reuse material, I made sure the person I was doing work for knew about it and the material was of equal or better quality than what was being replaced, and depending on the liability I sometimes got it in writing.