You can kill NPCs in Dark Souls, just as you did in Demon's Souls. Some people kill Lautrec early on, for example, to obtain his very useful ring. If you ever accidentally harm a NPC, they will be hostile towards you, but you can "absolve" your sins at the NPC "Oswald of Carim" in the Church after the Bell Gargoyle fight to make them unhostile. The cost is Soul Level x 2000. Some covenants will also become hostile if you dishonorably leave them (abandon covenant without use of Oswald, betray their rules, etc.), which is also something that can be absolved at this NPC.
Unless you kill Oswald, you can always resort to this method to make the NPC's like you again (note that this won't revive them however if you killed them).
You might miss out on some story/sub-plots, but you should still be able to complete the game having killed all the NPC's that you are allowed to kill. It is hard to say for certain what the impact on the game will be as there are so many NPC's in the game with different functions.
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.
Best Answer
If you're talking about a strict PvE build for your character, it's not that you'll mess up a build. You'll more likely paint yourself into a corner by dedicated leveling up of one stat and then, when getting to a boss, find that you put too many points into something when you should have been getting a bit more of another thing.
I wouldn't worry too much if you had problems with Bell Tower Gargoyles. It sounds like you're new and are getting used to Dark Souls and how it plays. Just continue to level up what you think works. I could tell you what would work for your character and what stats you might want to focus on, but you're more interested in playing the game without guides, it seems.
Continue to level up what you think you'll need. The game is harsh and will teach you what you need at certain times. Don't feel bad about going to a safe place next to a bonfire and grinding out souls against enemies that are easier for you. You'll find a few places in the game are useful for this (Forest after the gate, Anor Londo Silver Knights) to get the souls you need for a few levels in something to even you out. While the game is harsh, it is fair and won't make it impossible for you to continue on. You'll just have to take some time, think things through, train if needed, and breath.