According to an announcement from the end of last year, it seems this business has closed due to the owner’s retirement:
As to the general question, it is of course a compromise whichever path you pursue.
Selling piece by piece yourself, of course (assuming there is public interest in each item, which there may not be for some items) will most likely bring you the most money, but it will take forever, and there is a risk some items may not sell at all, depending on condition, etc (while you mention photographing items, you don’t really say what condition the trains were in… as an example “oh, it only has 2 or 3 places where the rust went all the way through the metal” does not equate with condition most people would be interested in today. Hopefully the train guy helped you when organizing to understand conditions somewhat). You certainly don’t want to do this if you’d like to be done with this project in a reasonable amount of time.
Anyone giving you money for the whole collection will result in it all gone in one action with nothing more to worry about, but you will probably get much less money for it. While they may make a good profit on some of your items, they also may end up having some of them stuck in their inventory for quite some time for any which end up with little or no public interest. You are effectively paying (by taking their offer that may be quite a bit lower that what could potentially be realized with individual sales) for their risk in exchange for them buying it all as a collection.
As mentioned above, the guides are not realistic in terms of what you can expect to recover when selling a collection these days. The eBay sold listings are probably more realistic, but you’ve already said you are not interested in doing that due to the time/effort involved. Assuming half of what you might see even in sold eBay listings if trying to sell as a single collection is probably overly optimistic.
You say your mom wants to sell the house and move in with your brother. Assuming she wants to do this in the near term, you are not realistically going to be able to do this in a timely manner without selling in bulk to a dealer or going the auction house route.
You mention “local train dealers”, but that you are new to this world (but did already hire the local train guy to help you for the cataloging you’ve already done). Do you know for sure that there are even multiple local train dealers where you are located? Many train stores have closed over the years. If you do really have multiple local dealers, you should probably take your list/photos to one or two and see what their level of interest is.
As to the auction houses, I don’t want to list other names, since I don’t know if they are CTT advertisers or not, but there are a few others not mentioned above. A big part of that equation is where you are located vs where each auction house is. If one happens to be truly local (within maybe a partial day’s drive), you may be able to do best at that route if you can bring the collection yourself (sounds like it would fit in a small rental van) to one of them. While most of them will pick up, they will probably also charge you a different percentage of the final selling price of your lot if they have to send out people with a truck to your location to pick the train up.
If you do consider the auction houses, most would agree that you should go with one that runs it’s auctions on-line (or a combination of on-line and in-person). In addition to eBay, you can try to see what similar items sell for at these on-line auctions, just don’t forget to apply a reduction for some percentage that the auction house will charge you for the sale. (which now that I think about it, you also should be accounting for when you review eBay sold listings, since as a seller you would be paying eBay fees on both the auction’s sale price and also for the shipping charged to the buyer)
Unfortunately there is no magic answer that results in the most money, in the least amount of time, also with relatively low investment of your own effort and time to prepare for an attempt to sell.
As painful as it is to think if “giving the collection away”, any alternatives can easily more quickly eat away at whatever money you are trying to get out of the collection (from paying for storage, delaying selling of the home, etc).
It’s unfortunate, but the “investment” many people thought they were making in trains is less than so, as the demand has seriously decreased from what it was 20-30-40 years ago.
The last unfortunate and sad part is that the market has lots of product from the glorious pre and post war train days right now due to the natural cycle of life. There are far more trains coming available than new people deciding they must have these older trains.
It is true that if you have these older trains in absolutely PERFECT (known as Mint/Like New) condition, there are still some serious buyers with both deep pockets and interest. Particularly if there are decent condition boxes to go with them (that’s one aspect f the collection you didn’t specify). Far fewer boxes survived from the pre and post war years than the comparatively rugged trains themselves.