That being said, I do lean more towards djuqa's opinion, although maybe not to the extent that he mentions.
You are perfectly entitled to believe what you want to believe, and to be honest to make great returns in any form of investing you have got be able to see the value that others can't otherwise you will be competing for assets that are already fully priced.
That said, you also need to remember when it comes to investing, it's important to see things for what they are, not how we wish they would be.
And I'm not saying you can't achieve a better result than my above approximations, but that will require additional time, marketing and negotiating and that needs to be factored.
Let's say, it takes an extra 5 hours of marketing and negotiating to strike a deal to sell UndergroundMining.com.au for $500 (which is much better than I think is likely) you need to factor in your time. And if your time is worth $40/hour that means that you have just sold the domain for $500 - $200 - acquisition/holding costs (I'm assuming $20)...
Leaving you with the grand profit of $280 on your $20 investment.
Now you might be thinking $200 profit isn't terrible, for a $20 investment (given that you paid yourself $200 to sell it) however that assumes that you will actually sell it (ie this doesn't factor risk).
For domains like this I think there is a reasonable chance that few if any retail investors will be interested which will mean you might do 5 hours marketing but still not get a sale, which will mean you have spent $200 worth of your time for no sale, and if you then have to liquidate the domain at auction, you might get $100 which will mean you lost $100 plus acquisition/holding costs...
So it's not as simple as saying I think this domain is worth $500, I'm rich.
@Djuqa - Thanks for the purchase offer, but I have a couple of connections in the industry, so might just shoot them an email to gauge interest.
At least counter offer him... You could get yourself an easy sale.
ie - if UndergroundMining.com.au is worth $500 to a retail buyer, and djuqa is offering $100, tell him it is his for $200. That way if he takes you up on your offer, you don't have to spend $200 of your time to trying to find a buyer, nor do you have to carry any risk of making a loss.
When it comes to domaining, a bird in the hand is worth 5 in the bush.
Or should I refrain from talking about offering it to other companies?
I'm terrible at sales and negotiating, but my first thoughts would be that you are generally better to say someone else has expressed interest, rather than you saying you will offer it to others.
One can be construed as a threat, the other is you being friendly enough to offer them the opportunity.
Should you set a time limit for the offer?
Maybe, but be careful about saying things you don't plan to follow through on, you will lose the power in the negotiation if they call your bluff.
Should you take offers via email, or direct them to NetFleet listing?
Depends - are there lots of other better domains on NetFleet?
Directing people to NetFleet can be good when they are worried about paying more than reg for a domain, they get to see lots of people spend big dollars on domains, but it can be a double edged sword because they will get a pretty good idea of what a domain is really worth, which might not be a good thing if you think their pockets are deep.