And be aware of unscrupulous sharks that are out there, now that you have posted on here I would put money on someone getting in touch and trying to buy it for well below it's true value.
That is a tough call for you to make as well, your chances of selling as a bulk lot are minimal so you may be faced with dribs and drabs sales for a long time, did your dad have any other railway friends or belong to a club or group? Perhaps somebody there may be able to help if so.
Do you have a model railway shop near you? perhaps you could post your approximate area and somebody could put you in touch with a local reputable honest model railway shop who could sell it on consignment for you.
Selling Ebay can be a bit hit and miss but some unlikely trains are going for ridiculous prices lately, but that could be a long process as well if you have a lot of trains.
But a bit of advice would be not rush into it, do some research on locals sources such as I have suggested, catalog all that you have with a detailed description of what you have, ie, don't write "yellow diesel with 8 wheels, new in box", the box should/will have information on it, so you may have an "Athearn (the manufacturer), Union Pacific (the name of the railroad on the engine), GP40 (the model number of the engine), all this information means a lot to us modellers, and state if its New in box or used, modellers will have a fair idea of it's price.
You won't get new prices for virtually all of your dad's trains, that is just a fact of life, some collectable stuff may be more valuable, at a guess, I'd say 50% of any new marked price max.
If you can get it all listed a good hobby shop should be able to give you an idea of what you likely to get. If you have a list and an idea of prices then you could advertise on here with contact details and the good guys will pass that info on to any club they are involved in, unless you don't want a squillion phone calls
And lastly, when I say model shop, I mean a shop that deals specifically in model railways, not Toys R us or one that only sells model cars or hairyplanes, once again, get back to us here and somebody should point you in the right direction.
Sorry to hear of your loss, and personel issues can be hard to deal with at this time, so I hope we here on Railpage can help you out.