You are only looking at the scenario from your (the buyer's) perspective. Yes, in both cases you are spending $10 to acquire the card. However, both sellers (who may also be reporting card prices to TCDb) still have to pay for postage to get that card to you.
Seller #1 offered no cost to you, but they still had to pay $5 to ship it, so they only made $5 profit on the sale, therefore, the card is only "worth" $5 to them.
Seller #2 asked you to cover the cost of postage + $5 for the card itself.
This can lead to several different reported "values":
$5 (if postage is accounted for and removed from the valuation on both sides)
$6.25 (if all parties in each transaction report perceived card values, minus the postage they paid = $10 & $5 [buyer], $5 & $5 [sellers])
$7.50 (if only the buyer reports perceived value)
$10 (if all buyers & sellers report the total of card price + postage)
This would only get worse with larger orders. What if you bought hundreds of cards or a complete set? Do you apply 1/800th of the postage costs to each card? If you request Priority Mail delivery, does that improve the value of your card? If you buy a card at a show or LCS, do you include a portion of the admission price, parking fees, gas money, etc?
Pricing of cards is a "ballpark" figure (pun completely intended), which is why so many members choose to ignore or not strictly adhere to the reported prices, here & on other sites.
Report prices as you see fit, but remember that others may not agree with your (or another member's) assessment.