My FIL asked me to sell his trains before he died a few years ago.
He had a fairly large collection of Marklin HO Model trains, which were all in their original boxes, perfect shape, etc.
https://en.wikipedia.org/wiki/M%C3%A4rklin
I started off as you would expect: E-bay, web sites dedicated to Marklin Trains, etc. I even stopped by a local hobby shop and chatted with the very knowledgeable owner. It became clear to me that the collector train hobby is a world without end and that trying to value my FILs collection was going to take some doing.
In my case I got lucky. The hobby shop owner knew the "best " Marklin guy and gave me his name and number. He turned out to be a retired Navy pilot and we hit it off immediately.
He and his brother ran the business and they had a very extensive web site. I spent days on it and other sites developing an excel spreadsheet, etc. It was a great winter project.
He happened to be visiting his brother in DC and agreed to drive down and appraise the collection at my house.
It was a great experience and I learned a bunch, especially the fact that with few exceptions, trains just aren't that valuable. My FILs collection was worth a few K and the bulk of that was in two specific, rare locomotives.
Sorry for the long winded post but start on the web and try and find the right person to guide your evaluation and sale.