When you seek at eBay Made in China parts for a project, what you should prepare your self this is to get scammed.
You first problem appears to be unreasonable pricing for low end goods.
The second problem this is manufacturing quality scam, and this starts regarding the quality of copper alloy that a banana plug this is made of.
The third problem, this is poor manufacturing design at moving parts as is the banana front isolator tube and how well this is attached to metallic nut - ring.
The first sample here this is the cheapest 4mm female banana plug for panel installation.
This is made of very low-quality Copper Alloy, inside hole this seems narrow and a male banana will get in less than 40% of total length.
Female banana body this starts as a metallic tube, at the end of the hollow part it starts the screw end, what I did discover this is that when you are tighten the nut the master break apart for the hollow front part.
The second sample this is the higher quality banana.This is higher quality regarding copper alloy in use, but this has poorly manufactured insulator cap and metallic ring.
The metallic nut this has anchor fins at it exterior so the plastic to hold on it.
Most reliable banana plugs makes in Taiwan they are thermally bond to this metallic nut with the outer plastic isolator (cup). The Chinese factory instead it did use only 25% of required metal for the construction of this metallic nut.
The outcome it is obvious, all such cup insulators they get off the metallic nut and they become useless.
Naturally some one will think and say that you may use a strong glue and fix them, but this is not the problem, the problem is that the cheater will scam several hundred of people by making a fortune and by wasting our time and money.
Yes the first sample this looks to have consistency of 75% recycled aluminum mixed with 25% recycled copper.
The better sample this seems to be of Brass that is an alloy of copper and zinc, mix proportions unknown but less copper there too but not recycled metals in it.
Metal contactivity this is acceptable (end to other end milliohm measurement).