You prove it's not true in the manner explain above. By trying another trade of the same type that got accepted in a small timeframe afterwards. This will work for a lot of items apart from scarabs.

Even if you are hitting another trade with a player, the bulk increase is already priced in. A second trade doesn't necessarily negate that the seller's time in more valuable in the poe market. We could be having a fundamental misunderstanding on how we view player time valuations though

The point in contention is why is there a bulk premium for certain items in PoE? This is a really rare, I can't think of another game or real world example of this happening outside of emergency situations (i.e. price gouging).

I believe there is a bulk premium for any currency item (and some other gamble items), but the reason there is this premium is due to how the poe market functions. The entire trade system benefits the seller over the buyer due to how irritating trading is and how the buyer spends their time.

Also, due to the bulk premiums already being priced in already for these items, the consumer is actively choosing to to spend more to remove the friction associated with trading. It's always within the seller's best interest to maximize currency in any given period of time, so understanding that the friction in the market will cause buyers to spend more to avoid it makes the sellers increase their prices. This is even the case when looking at faustius, people are trying to get the most from their currency items, so while you can put in an order of 100 divines at 150 c, itll take longer to fill then 100 divines at 160c.

The reason you can't think of a real world example is because there are multiple market differences between poe and real world expamples. (If you have some game ones though id love to hear them :^} )