A person who collects beetles is a coleopterist.
A person who collects shells is known as a conchologist.
Numismatist
hemipterologist
a person who collects stamps
A person who collects things can be called a collector. If the collecting is thought to be excessive, such a person might also be called a packrat, or a hoarder.
A person who collects nests or eggs and studies them is called an oologist.
A geologist is a person who studies rocks and rock formations. A rock collector or a rock hound is a person who collects rocks for fun.
Someone who collects everything would be called a hoarder - this type of person may have OCD as well.
There are many ways to describe a person who collects things. They can be called a hoarder, a magpie, a gatherer, finder, or a hobbyist.
Some of the names given for a person who collects trains:model train collectormodel train enthusiasttrain modeler
philatelist