Investments are for the stock market or property etc where you have an excellent chance of making a profit. Some toys will appreciate in value if they are rare and old enough .. the seller will also need to be old lol.
I buy to have enjoyment playing with Marklin .. don't even understand collectors who put trains in a cabinet complete with a sound decoder which will never be heard. However we are all different and have different views but I just like the fun and enjoyment of model rail.

I have never had a layout. When I was a kid, twice a year the house was mine and my trains, the tracks were running from one bedroom to the other, to the living room via the dinning room via the hallway, I guess, that there were about 100m of tracks and many meters of wires to power the switches and the home signals. The 3025 model is fast and noisy, a sheer delight . It went up and down the bridge and took tight turns at high speed until it was derailed one too many times and broke into 3 parts , so I have exchanged it for a flash light (made by Metz, still have it) for my camera.

I grew older, moved around and developed other interests and hobbies and the trains remained in the boxes, yet, from time to time I bought more, then some found there way into the coffee table .

The options are:
1. To sell at current market price (have no idea about Kohl values).
2. Keep and look at them from time to time.

If I get more realistic prices then those on Ebay I will let them go, otherwise they can stay where they are.
I am not hungry or desperate for money, the models don't eat or cost money, so why to sell? I can look and enjoy them behind the glass...where they are for the last 25 years.

I doubt it will be a good investment any more. The rise of internet sales have made pretty much all Marklin trains on offer in the world available to us, so supply went up. Add to that the (perhaps sad) reality that many collectors are no longer with us (to use a euphemism), increases supply even more. The demand, for the same reason, is down, so overall I would expect prices to drop.

But the silver lining for ME is the hobby is more fun, because I can now buy a nice, 10-15 yr old loco for around 100 E. The 'hunt' for a good deal is certainly part of it!
But I doubt it will return much when I either decide to sell them, or want to cash in.
I can only echo the words of most responders here, that it is simply a hobby, and that hobbies are worth spending some money, if you really enjoy them!

Currently my oldest loco I got from St. Nicholas when I was 5 yrs old is on my table to be restored. I am afraid that will cost a multiple of what it was ever worth... For me it is a good investment, though, because I will never ever sell that one!