You have probably had very bad luck with your invasions, as people waiting on spawn points are 99% sure gankers. Lots of people start playing in "creative" ways such as this one when they finish the game, just to have fun.
When you kill an invader as a host, you get 15% of his souls the host needs to level up, and when you kill the host as an invader, you get 4% of the souls.
You don't have to feel as a jerk, invasions are part of the game, even of the lore (Darkwraiths, etc.). As a host, I've got very annoyed with invaders when I have a large bunch of souls and I know I'm near a bonfire and they get to kill me, but you can't evade it. As an invader, lots of invasions are full of white phantoms, just keep in mind that they might only want to do PvE, and then there you come! I'm an honor guy, so when 4 people attack me at the same time, and without even bowing...
P.S.: Having played all the Dark Souls saga intensively, I have to say that this one has the most troll PvP I've seen.
Alright so I did some digging using a friend, CE and the latest table(not linking it because it's Bad Stuff™).
CE Values
The table I used didn't list item discovery directly so I had to search a bit for it. As expected, it is affected by luck and item discovery gear. Having another player didn't change it, and having the other player equip item discovery gear didn't change it either. Now I'm not a pro at CE but I tried to find places that used the item discovery value and what I think was the drop rate formula didn't have any changes when another player equipped I.D. gear.
Ingame observations
First, I did 5 "cycles" of killing the 3 Silver knights in Anor Londo with no item discovery gear and noted the drops. Then I did the same with my friend, no item discovery gear. It didn't make any difference.
I did 5 cycles with gold covetous ring+3, Symbol of avarice and a crystal sage rapier and noted the drops. Then I did the same with my friend (who had no I.D. gear). No difference. He then equipped the same stuff as me. 5 more cycles, no difference. We did this again a couple of times with him as a white, gold, purple and red phantom and it didn't change anything.
As a last attempt, I used the Dried Fingers and summonned the max amount of players I could and got invaded. I killed the 3 knights. No noticeable difference in drops, no changes in CE.
Conclusion
Extra players of any kind (white, gold, purple, red), summoned or invading, do not appear to change item discovery or drop rate.
Best Answer
If you plan only to transfer souls, look for items which cost 500 and could be sold for 100 (5:1 seems to be the best possible rate), some of these are:
I think best investment for current character will be Titanite Chunks for 13000 souls each from Shrine Handmaid (both Titanite Scale and Twinking Titanite are more expensive, but mostly useless in my opinion - as boss weapons suck). Unfortunately, these cannot be traded.
Beside upgrade mats it makes sense to stock with all useful consumables - Green Blossoms, different resins, firebombs.