I think when the server detects you are going to run into inventory issues when trading, it should not allow for a trade to occur from the client that is trying to trade. I really think that should be fixed considering there's no reason that a player would even want to continue trading knowing that they could run into stacking issues.
Quality-of-life suggestion
There could be a little popup that denies trading that simply says something like:
"You will run into inventory issues with this trade, please reorganize your inventory to make space before creating trades with this many items."
I also think that MarkCX should be cut a little slack due to this weird popup (it should just disallow the trade if we know it could cause issues) and should possibly be given a refund whether it's NPC value of the items or whatever.
Roughly calculated prices if a refund could be processed:
(whether you're calculating NPC prices or the marketplace's value)
20x Gold Ore
Marketplace Value: $1,100,000
NPC Value: $200,000
12x Chemical Tokens:
Marketplace Value: $2,640,000
NPC Value: $0 (could be argued that it's worth the amount of chems you get per token)
214x Gas Masks:
Marketplace Value: $9,416,000 (a bit iffy to refund this amount since it's unlikely Mark would've been able to sell all of these for the marketplace's value).
NPC Value: $2,140,000
8x Diamond Ore:
Marketplace Value: $4,000,000
NPC Value: $240,000
43x Legacy Forger:
Marketplace Value: $6,020,000
NPC Value: $215,000
Hypothetical refund amounts:
(whether you refund for the NPC value or marketplace value)
Marketplace Value:
$15,900,000
NPC Value:
$2,795,000
Median Refund Amount (between both value types):
$9,347,500