Hi Caroline.
Yes, ebay is good. Last time I checked you could post a large number of lots per month without paying fees unless you sold. Make a few listings to get your feet wet, and then make a few more. You can have some marbles for auction and some for buy-it-now.
If you mix it up and sell some small lots, some medium lots, and maybe some individuals, you can probably get quite a few people checking on auctions and trying to win several of them to take advantage of combined shipping. The last time I sold was almost four years ago now but I set a modest goal for myself of listing one or two new groups a day and I was able to keep pretty busy with sales.
That won't get you as much money for them as a longtime seller with great marble knowledge and a lot of longtime customers might get for selling the marbles one at a time, but it's a way to get a pretty good amount of money. Pretty sure you would get more that way than if you tried to sell it all as a big lot. You'll get a lot more potential buyers if off the lots in affordable quantities. Could be a long wait to find a buyer with deep enough pockets to make you willing to part with the whole collection in one lot.
But now that I mention longtime sellers -- and Al mentions Dani -- Dani has been known to sell on consignment. So that's another option you might looking into. Al, do you know if Dani still does that?