Why not limit this to a warning? Instead of prohibiting trades as detailed in the first post just have a highly visible warning every time a trade would violate the propositioned rules. E.g.: "Yes, I've read and understood that this trade with $50+ worth of cards is with a user with 0 feedback and I'm at risk here if I don't get him/her to send first. Click checkbox." That would kind of get the best of both worlds, you'd grant an extra level of security, since, let's face it, most scams come down to social engineering anyways, so a simple warning is actually a real security asset. (There's a reason why banks repeat over and over "don't give your PIN to anyone".) But at the same time, you'll allow people who are willing to take risks the freedom of trading what they want with who they want.
Some comments about the specifics of the rules, rather than their general nature:
Is it really intentional that level 1s are unable to trade with level 4s? Why would I ever want to be a level 4 (instead of level 3) trader if that meant that the vast majority of traders wouldn't be able to trade with me? Apart from lifting the $250 ceiling, that seems like a downgrade.
I understand that the value limits are arbitrary and, no matter where you put them you'll end up with some awkward results. But with current values you can get Marsh Flats for just under $50 and Verdant Catacombs for over $50. Similarly there's Jace, the Mind Sculptor for just under $100 and Liliana of the Veil for just over. This ends up feeling very strange, where, if you want to get rid of your BW tokens deck for a Jund deck in modern you can trade off your marsh flats, but can't get Verdant Catacombs. It just feels weird, and would ultimately make deckbox a much weaker tool Since the "trading opportunities" is one of the strongest features here and I really don't think it'd be good to limit trades along such arbitrary lines.
Finally the good stuff: Limiting the highest level to users with an active and verified credit card or paypal account seems like a good idea. There's a ton of safety in connecting the accounts to actual, real world, IDs. Espescially because this limits the swapping and borrowing of accounts. Even though I'm not a premium user I support this part of the idea.