I really don't care for the initial proposal. I am very picky about the trades that I do (and I am a collector so I keep a lot of stuff) so I have been on the site for 2 years and only have 13 trades.
3 of those trades were purchases before the marketplace existed so I have a 0 value trade for them. My most recent trade was a Portuguese box of Rise of the Eldrazi for some duals and other cards. That was a $250 trade, but it shows zero on my side. Plus I have a number of trades where the value has significantly decreased from what I sent them as (standard stuff mostly). If you were to add everything as it happened it should be well over $800 in trades/purchases for those 13 positive reviews I have.
The value shows as ~$280. I feel that I would be negatively impacted by this.
Also with the proposal, a level 1 user wouldn't have been able to trade their prerelease foil Narset for a bunch of standard cards.
And finally I can remember a lot of users with high rep scamming (or at least providing poor trading experience) people back when the disputes were in an open forum. I give the benefit of the doubt to larger users and I always offer to send first, but I am just as concerned with being ripped by a high rep (>20) vs a lower rep (5-20).