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
Yes, but not for Chaos Servant.
The following are the covenants which will remove your access to spells upon betrayal:
It's also worth noting that betraying the Gravelord Servant covenant will cause the miracles Gravelord Sword Dance and Gravelord Greatsword Dance to deal reduced damage.