This will get all items in the user_items table belonging to $otherusername.
$sql = "SELECT item FROM users_items WHERE username='" . $otherusername . "'";
$res = mysql_query($sql) or die(mysql_error());
To sell an item, you would have to INSERT the item into the user_items table. I'll leave that as an excercise for you to figure out. Basically look at the SQL INSERT command.
$sql = "INSERT INTO user_item (col1, col2, ... colN) VALUES (value1, value2,... valueN)";
$res = mysql_query($sql) or die(mysql_error());
Read about the sql INSERT command here:
SQL INSERT