everyone can always buy
to help out if you're struggling to farm. (but i dont recommend it, it never pays out in gold before i need the item slot)
but if you are GP and just getting harassed into oblivion....
thats the point where you stay under your turret, and allow your opponent to free farm so you to can get farm when it eventually reaches your turret
if you need mana try getting that sheen for your triforce.
if you mismanaged your wave and the enemy is freezing it near or around their turret your lane is over, your only hope is to wait it out till 6, and ult and force the wave into enemy turret and reset the wave so its in the middle of the lane again.
take gromp/golems
Q farm, you still get the exp and bonus gold.
if you are under your turret and cant last hit the minions you need to "prep them" meaning calculate the number of autos you need to kill it so it coincides with turret damage, so you can effectively kill it with an auto attack (mages have to do this all the time)
https://www.youtube.com/watch?v=ad8wK1OiTsQ