if the net is on for more than three other lower prices, it wont even show up on the list. More importantly, if there is a single net price that is lower than yours, you wont be able to buy yours.

that would explain it, o why is it i can try and sell a net for 500 but i cant sell other items for more than 30? 

you can sell items anywhere above 50% its orignal price and below 3000%

just to clear two things up that jb said...
1) non-subs can only sell up to 2500% on market (subs can up to 3000%)
2) if there are too many priced lower already, it won't let you offer it for higher.  the market can only hold so many of each item

--jason