While the tool has never been used, I don't think that's as ideal of a situation as one that has been used but was well-kept by its owner. It's a mechanical piece of equipment and it's been sitting for (ostensibly) 18 years. Like a car that's been sitting, I'd be concerned that seals and lubricants may have dried and maybe rust or corrosion may have occurred with the internals. I would not be willing to consider paying $900 for an 18 year old tool (presuming that this is a first-generation tool - which the classic systainer suggests) that is well out of warranty and who knows how much it might cost to send it in to Festool for service work should there be a problem. To my mind, $600 would be the absolute most I would consider paying.
That said, price flexibility will depend on the buyer and their knowledge about the Domino (or Festool, in general). Some positives are that it is a very clean example and the systainers look to be in great shape. The tenons look to be in good condition, but we recently had a poster complaining about swelling tenons, so that might be an issue with 18 year old tenons. The tool probably has the locating pins versus the current flaps - but some users seem to prefer the pins, so that could be a plus for some buyers.
I realize that my position is starkly different that the others, but I've spent the past year wading through quite a bit of used Festool offers. I think most sellers are unrealistic with their prices and the people who are paying some of the asking prices just don't know enough thinking that any amount off of MSRP must be a "deal".
But you want to get the most bang for your buck, so price it as others have suggested and see what you can get. Someone out there will probably think it's a great deal. If interest is mild, consider your cost. You said you got it as an estate sale, so I presume you got it for a song. If it doesn't move quickly, be open to offers. Consider your actual cost and if an offer comes that gives you a decent return on that cost, sell it to them. You get a good return and that buyer gets a good deal.
And I agree with ChuckS - the new, cordless Domino is coming. You probably want to sell this one ASAP, because if I were in the market now considering a new DF500, it would be for the cordless.
Good Hunting!