At most probably $650-$700. That's giving you a pretty hefty inflation.
The 6970 is struck down by the 7870 but still not super old. It'd probably be the most expensive part of your build $300 @ most. The i5 750 is old tech and the i5 2500K beats it by a large margin. Getting $120 is pretty good considering the i3 2100 is at that price but has pretty similar performance. The 1TB still costs a lot since the price hikes. But yeah, you may be able to get $700-800 depending on how ignorant the buyer is about computers.
Hi, "I thought it could be worth up to $1000? (canadian d.) "
For used, figure nothing for the assembly labor. Used part prices will total higher than total system value. Not sure you get anywhere in the neighborhood of $1000. Unless you have a friend who knows the system and wants it.
Stepping through the parts:
case, mouse, power supply, keyboard are part of the system being sold ? = $100
it's a i5 750 = $135 USD (used, ebay)
HD 6970 2gb = $250 USD (used, ebay)
p7p55d-le = $100 USD
dvd burner = $20
1TB to 7200 hdd, = $50
windows 7 64 bits, = $50
4 gb ram 1333. = $50
Maybe $ 600 USD? If you sell the video card separately the rest of the system will sell for a bit more combined.
Aside: this is such a strong system I'm not sure how much better you'll do when you make your own. The i5-750 is not holding you back in games, nor is the hd6970. Consider incrementally refining this system, maybe OC'ing the i5-750 if you get bored.
aside2: USD and Canadian dollar almost the same today.