So youre the ADC marksman and your support built full AP and stole all the kills took a lot of CS...
Dont act an ass about it. Yeah, moan and complain a bit sure, but dont just stomp your feet, shit into your hand and throw it against the wall, and give up cause youre not going to get to carry probably.
Now higher skill, higher game knowledge players know how to take lemons and make lemon-aid. Most of you get lemons and try to make arsenic... grow up, dont be a pathetic tantrum throwing child, the girls dont think its cool.
Heres what you do... I know it will sound strange, but here me out.
First, stick with the team, dont risk your life, just try to tag the fight the best you can near the most fed person. This will get you some assist. Save your flash to get away, not to kill. You need to stay up so when the bodies fall, you can farm minions. Watch the death timers. When the enemy has recovered or that assassin thats eating your face off is back up, return to safty mentality and try for some assist.
Stop building damage!
A. You dont do any your behind, so more of very little is still very little. B. You are probably easy to kill, and dead people dont get gold.
So being squishy and doing very little damage just makes you a bag of 300 gold for the enemy team to take, making you hurt YOUR teams chances of winning.
So build what?

Frozen Mallet will do 3 things for you.
It will give you some extra HP for survival. It will give you some advantage when trying to escape/kite away. It will let you perma slow an enemy champ for your team to kill getting you assist (aka gold).
Guardian angle makes you more durable meaning you wont die as easily to burst, and if you do, you still may not feed that kill. It also means if you are targeted with a dive under a turret or something, the diver may still die, getting you an assist.
So with all these assist and some farm, youre still behind, but have gotten some gold... can you get some more utility, or contribute in another way?
YES!
Grievous wounds is a pain for many champs. Cutting down on any healing theyre getting can help your team put them down.
Youre thinking "Straight damage item?" Well yes, and no, the active will cripple the move speed of an enemy letting your team chase them better, and the bonus damage could even get you the kill.
So you may also be thinking "This is a terrible marksman build, wont I be made fun of in chat by my team?" Maybe, just say youre behind and trying to contribute the best you can with what little gold you have. If that doesnt work, mute. About 25-40% of the players in this game (depending on time of day) are self-absorbed brats that wouldnt dare act the way they do in the game cause they dont want to be missing teeth from all the people around them they would be pissing off. Ignore them, they arnt worth you time.
Now because some people wont read this with the proper frame of mind and probably think Im saying "Build this, its will make you awesome" IM NOT SAYING THAT.
boots (prferably
or
for escaping) , and maybe even one of these
. Most of these have some defensive options, or in Wits End/Black Cleaver case offer a way to allow your team to do more damage while giving you defensive stats. Iceborn G. Will give you more CC, and paired with Frozen M. It can let you slow more than one person in a teamfight. M. Scimitar will let you un-screw yourself with its cleanse effect.
These are options that when put together may help you contribute to a victory when youre very behind or maybe even cant get any teamwork/support/peel and are targeted in every fight. These items still have to be paired with cautious gameplay and attempts to group with the strongest team mates. You wont carry with them, but you can stem the flow of gold you would give to the enemy team, and maybe give your team advantages to help them win.
Good luck, I hope you dont have to suffer a CS stealing, kill stealing, zero warding, zero peel, full ap, no sight stone support that puts you in a hole. If that does happen, consider some or all of what Iv said here.
is only 25 AD and is by far a cheaper and more cost efficient item that can build into
).
or some utility like
). But just not building damage and getting a