I agree this would be a good way to make a bid system less frustrating, most MMOs with bid systems function this way. Bidding still remains a very uncertain and inconvenient way of trading, though. While it would be very effective in keeping the power of Trade in check, it would also make it kind of an unreliable non-factor in progression, which again begs the question of why you’d have it at all, then.
This just goes back into why Trade is so hard to balance in an ARPG. It’s hard to compare to MMOs with full-fledged economies, because economies fulfill more functions there than just gear upgrades (mats, professions, cosmetics, pets, mounts etc) and currency has sinks in other aspects of the game. In ARPGs the entire game is only about loot-hunting and progressing, so Trade will always sit on the fence between irrelevant (because too inconvenient/unreliable) and too efficient/powerful (because farming currency is more efficient than waiting for specific drops/access is too easy).
I’m very curious why you think loot is in such a bad state, or rather what you think is missing. I don’t think the current structure is going to change drastically until release. Sure Legendaries are coming, however those turn out, and i’m sure more unique items and probably more bases and mods, but other than that I don’t feel there’s a lot missing.
Your notion of “fixing” loot sounds like you want way more diversity and more stuff dropping, (maybe comparing to current PoE?), but if you recall early PoE had very similar loot variety and density, and more mechanics added more as time went by.
As I don’t think loot is going to change in any dramatic way before release, I think it’s entirely viable to discuss the ramifications of Trade, as it functions well in SSF right now.
I would expect, if Trade is implemented, that Crafting would cost gold for example to create some natural sinks. This would also maybe be a way to allow for better chances on high Tier crafting - just make them cost quite a lot of gold, so you then have an organic gold sink that also makes you think twice: do I want to spend it on a good chance to improve an item, or do I want to buy something from the marketplace.