(And if you're a purchaser intending to use the item, you know this is true and can afford to lowball the offer).
If this was true, then it follows that the PCs could lowball the offer on any item offered for sale (although not any item commissioned).
Um....no? They won't get the item if they lowball the offer, because the seller knows he only paid 20% of the price for it, and can afford to wait until someone with some real cash comes along.
If he paid 50%, that might be a different story.
Buying from a merchant and buying from some sweaty adventurer-type...not the same thing.
Sorry, but I am a bit confused here.
Why would a merchant who payed 50% accept a lowball offer, but a merchant who paid 20% not do so?
It is generally true, IRL, that you can shave cost off of more expensive items than cheaper items. For example, the last time I bought a major appliance, I was able to get the mail-in rebate upfront....and I was still able to claim the mail-in rebate. This is because I know that X% of the sale price is the commission, and a commission of X-Y, so long as Y is smaller than X, is still better than no commission at all.
It is hard to imagine how an adventurer could shave cost off of a few iron spikes -- where is the incentive for the merchant? But if you are talking an item worth (say) 5,000 gp, for which the merchant has paid 1,000 gp, it is not at all unlikely that he would accept an offer as low as 2,000 gp.
Again, in real life, every comic shop everywhere has "wall books". If you wanted to buy the most expensive wall book in any given store, there is a very good chance that you could get better than a 20% discount on that item.
Finally, it follows that if our end-uesr knows that adventurers cannot sell the item for its value, and can therefore lowball the offer, that our PCs could become the end users and do the same to other adventurers. They could even hire agents to do so, seeking out specific items, and thus pay no more than 50% of the value, after paying the commissions and costs of the agents.
RC