Foamy himself even said that he doesn't handle suggestions involving real world money as he has no experience in them, even Brad himself is against using marketplace. This argument has been brought up before and it has been decided, I'm done here.

Even if he doesn't directly deny the usage of Marketplace.tf as proof in that suggestion, there are still multiple reasons as to why it shouldn't be usable, besides that, there is nothing that supports the sales (@0.12$) that happen on marketplace anyways, there are not even sellers attempting to sell for 0.12$, the sales @0.10$ however, support my high-end.

With keys constantly raising and their cash market being relatively stable it is obvious that Refined Metal drops in price, as mentioned above, there is nothing that supports 0.12$ besides the sales at Marketplace.tf which should be unusable for of a multitude of reasons:

(1) Marketplace.tf has a very limited pool of sellers which leads to barely to no competition, therefore a healthy economy just can't exist on the site, people can list their metal for as much as they like and they will sell it eventually as there is no reason to sell for less, there is a huge demand for refined on Marketplace.tf (as shown by the sole amount of sales) yet there is very little demand for it outside of Marketplace.tf (which leads to it dropping, as sellers outside of marketplace.tf need to have very good prices to have any success in selling.

TL;DR: If Marketplace.tf would give the opportunity to sell refined to more people it would logically make sense to consider it usable as proof, however, if that'd ever happen, (which is unlikely) there will be competition on the site which will eventually lead to the market adjusting to the key price.

(2) The main reason people buy refined from the site is because it is the easiest and safest way to buy it, you can buy via whatever suits you best and there is never any type of fee (at least not to my knowledge), it's the total opposite of having to add a trader to wait for their response only to do a very slow and risky PayPal trade, with the only reason being a 0.02$ discount which is often lost due to fees anyways.

TL;DR: People are willing to pay more on Marketplace.tf as it is more convenient than any other way of buying refined.

(3) Backpack.tf is trying to capture the value of what items are commonly sold for among the community, with Marketplace.tf allowing few to sell their sales shouldn't be considered "among the community" (as it is only a select few which are able to sell on the site) which should make them invalid to use, I think the sellers @0.10$ (and even the 0.09$ sellers) struggling to sell any refined clearly show that 0.12$ can definitely not be considered a value for which Refined commonly sells for among the community. Even 0.10$ will probably end up being high, but the sellers did receive a few offers and the sales @0.10$ on marketplace.tf do support it so it is justified (for now). Logically 0.08$ - 0.09$ is probably the most accurate value, comparing the key/ref price to the price keys commonly sell for in money and the little success of the sellers @0.09, but the proof just isn't there.

TL;DR: Using Marketplace.tf as proof would go against the very basic idea of backpack.tf.

I hope I was able to justify my point and got it across well.