What I am wondering about is the price of stuff like antiques or used goods. I know you can pay a little less than the new cost due to how old the item is. However, I bought 3 icons from a garage sale for $5. One of them was hand painted in France in the 20th or 19th century if I had to guess. I am wondering if I should go back and give them more as it probably cost more than 5 bucks to produce that work of art to begin with. Another thing I was wondering is auctions. I'd assume paying low would be fine since the seller takes the risk upon his or herself. I remember reading Aquinas applied just price to just sellers and just buyers so it seems so as long as the two people are just and they agree on a price it could possibly be fine. There is a lot of nuance it seems. Another thing I'd consider is paying at least the cost something was to produce I'm a factory. I am currently looking for 80s instruments and this stuff crossed my mind.

Now according to these laws it is just for buyer and seller to deceive one another (Cod. IV, xliv, De Rescind. Vend. 8,15): and this occurs by the seller selling a thing for more than its worth, and the buyer buying a thing for less than its worth. Therefore it is lawful to sell a thing for more than its worth.

 5. Therefore if either the price exceed the quantity of the thing's worth, or, conversely, the thing exceed the price, there is no longer the equality of justice: and consequently, to sell a thing for more than its worth, or to buy it for less than its worth, is in itself unjust and unlawful.

if a man find that he derives great advantage from something he has bought, he may, of his own accord, pay the seller something over and above: and this pertains to his honesty.

  Accordingly, if without employing deceit the seller disposes of his goods for more than their worth, or the buyer obtain them for less than their worth, the law looks upon this as licit, and provides no punishment for so doing, unless the excess be too great, because then even human law demands restitution to be made, for instance if a man be deceived in regard to more than half the amount of the just price of a thing [Cod. IV, xliv, De Rescind. Vend. 2,8.
On the other hand the Divine law leaves nothing unpunished that is contrary to virtue. Hence, according to the Divine law, it is reckoned unlawful if the equality of justice be not observed in buying and selling: and he who has received more than he ought must make compensation to him that has suffered loss, if the loss be considerable. I add this condition, because the just price of things is not fixed with mathematical precision, but depends on a kind of estimate, so that a slight addition or subtraction would not seem to destroy the equality of justice.

 For sometimes it happens that the seller thinks his goods to be specifically of lower value, as when a man sells gold instead of copper, and then if the buyer be aware of this, he buys it unjustly and is bound to restitution: 

2) There's no principle that would stop a person from selling something for LESS than it's worth.  If I want to, I can give something away for nothing, or sell a $500,000 house for $1.  There's nothing immoral about that or about accepting it.  What's immoral is only if the SELLER charges more for something than it's worth.  This principle doesn't prevent a BUYER from paying more for something than it's worth.  If some price gouger wanted to charge you $100 for a roll of toilet paper, you can certainly buy it without sin, as the sin would be on his end.  And if he wanted to give it to you for free, there's nothing immoral there on either side.

In summary, if you can get a deal and get something for less than what it's worth, then there's no sin in buying it.