I would not call it "lowballing" but rather an indication what the stuff typically goes for on a bad day. Sure, all of us who ever sell something from our collection once in a while strikes gold and meets a buyer willing to spend virtually anything to have the item, but just because a 5150 once sold for $500 doesn't mean every 5150 will meet that amount.
I see too many ads where people have cleaned the garage and found an old computer which they try to sell for best offer, and specifically write that low bidders should not bother. Well, it is great for them if they get the right buyer, but I sense a fair deal of those computers end up at the dump three months later when no buyer was willing to pay what the seller had expected. It is in the light of this I would rather tell someone a 5150 with EGA monitor may be easily sold at $75, than if the seller tries real hard he will get those $500 after a while. If too many $500 posts show up and get indexed by Google, people outside of the vintage computing hobby will get too high estimates what most of us really would be willing to pay.
Then there are those of us who have the strong belief vintage computers should have practically no monetary value. It means vintage computers should be swapped around, but rarely or never be sold. I don't adhere to that "religion", as I quickly came to learn anything with a high demand but limited supply creates a market value, like it or not.
So to sum it up, when someone asks "what is this computer worth", they should rather ask at least four questions:
1. What is the most I can ask for it and be 100% certain to find a buyer. Sometimes it can be as low as $1, sometimes much higher.
2. What is the typical selling price on the open market?
3. What is the concluded market value? Often collectors value a computer slightly higher than it actually sells for!
4. What is the best price I could expect if I advertise it well and for long?
Then we have factors such as location, condition of the item, any rare or useful peripherals etc to weigh in. Often the "how much" people don't bother to list all that is included or don't know which items are worth mentioning specifically. Even if they supply an image of the system, it may take a trained eye to spot one particular piece of hardware or software. Then it is up to the person who replies to mention this particular item or not, a little depending on whether the responder would be interested in buying it or not. Yes, it would be a very sneaky thing to do, to give a fair quote for a base system but secretly knowing this one has something extra, but then again we all are humans, who have evolved through thousands of years by being sneaky to eachother.