There is an NPC, Kingseeker Frampt, that will consume your items in exchange for souls. He appears near the Firelink Shrine after you have rung the two bells. So yes, there is a way to sell item, but not until later in the game.
However, there is almost no limit to the number of items you can have in your inventory (see this question Is there a limit to the number of item in inventory?). You can hold up to 1984 different items in your inventory, and if you purchase the Bottomless Box, from a merchant in the Depth, you can store all those items in it and have a manageable inventory.
However, I think you should just drop the duplicate items because the resell value is really low. This table, from one of the Dark Souls wiki, shows how many souls you'll gain from each class of item:
Weapons 50
Armor 100
Dung Pie 200
Cracked Red Eye Orb 500
Copper Coin 1000
Some unique items 5000
Even if you resell 1000 longswords, you'll only gain 50000 souls. Compared to the time it will take you to sell all of them, and to manage your inventory, I think it is not a good trade-off. There are quicker ways farm souls.
During your second play through, reselling some of the unique items may be a cheap way to get some souls. However, reselling basic items does not seem interesting to me.
It could be the controller as I have personally had many old controllers like the thrustmaster and honestly found them to be pretty bad, I cannot really answer if it is the controller as you are the only one with your controller, if anything look at it in the settings (Control Panel > Controllers) and see if the controller reacts to button presses quickly and properly (As in pushing forwards and then letting go doesn't cause you to take a step back.
The second thing is it could be dark souls, dark souls has a certain timing to many of these attacks that take time to get use to. There are delays in swings because your character has to draw their sword back or pull their foot up and kicks requires specific timing.
The timing for the kick is to press the attack button and forwards at the exact same time. Not one slightly ahead of the other but nearly exactly at the same time, the reason is sometimes you want to move forward and attack and not kick so the timing is specific to ensure you kick when you want to kick and attack any other time.
Secondly the plunging attack requires you to not jump off or roll off initially. I don't know if you're doing that but it will delay your attack. Just walk off the ledge at the boss and as soon as you start falling hit the attack button and you should perform a plunging attack. It can actually be done a little too late and seem like you'll miss it only for it to land.
I use a PS3 controller with a tool to make it function like a XBOX 360 Controller and the game reacts perfectly too it and I can perform all these attacks just fine.
Best Answer
It is absolutely possible to get comfortable at the kick and jump attack but there is no setting to make it easier.
I am unsure of the frame data for kicking, but the active frame window for parrying (one of the most advanced combat maneuvers) is from 6 to 8 frames, I assume it is the same for kicking.
That being said, the game runs at ~30 fps giving you a whole 1/5 of a second to input the commands together.
The 'forward' command is inputted at the top of the joystick gradiant meaning that you can press forward but it will not count towards the kick command until it hits 100%. Because of this, most times that I fail the kick it is because the forward command was too late. I have found it is easiest to input the rb when I hear the 'click' sound of the joystick hitting its cage, in an effort to delay the forward command until I know it is actually being inputted.