Looking for trades, i see the trade value of my next drat picks is going down : 684 today, 903 yesterday, more than 1200 like three days ago.
What's happening ?

Winning those preseason games, which really shouldn't factor in imo.

Then, i have torebuild my team, and i'm not able to make trades :(

That pick trade value will fluctuate throughout the season based on your record, if you finish sh with a poor record and thus a higher pick that value will go up from where it is at now.

I have lost my last Two preseason games, and the value has stayed the same. It doesn't seem logical. Still normal ? I would have expected that value to go up.

i wonder if player improvement roster wide has any effect?

some kind of roster rating ? It's supposed to be based on record (ithought it was the previous season, at least for a few weeks), so i don't think it is that.