Love it. I've sold pretty much all of the upgraded cards I got from level 5 in order to make various decks that stroke my fancy.
I think 2000 score is high enough that people wouldn't make multiple accounts just for the 500 score quest, but if so, 2500 should definitely do it.
I don't think making the weapon untradable is possible, or even desirable. There's probably no record anywhere of which weapon you chose originally, and futhermore, I think it would be nice to allow people that chose a weapon originally that they no longer have a use for the opportunity to trade it for a new one.
Such a system could be very easily implemented. The easiest way that I can think of is inspired by playing runescape almost a decade ago:
you type in the person's name (like pvp dueling), connect, open the window with all of your cards that aren't in your deck, and a central area that clearly shows what both of you are offering. There should be a little tab to offer electrum as well, and a box to chat back and forth.
When you've both finished putting up your offer, you click "finish", and the computer checks to make sure everything is kosher, then puts up a "confirm" button with a checkbox next to it, to encouage players to check carefully to make sure everything's in order before they trade.
I'd really love to see this implimented, since it would give this game the one missing option that stops it short of being better, imho, than many of the popular TCGs of the day.