i thought i would be able to help, i know some people that deals skins from different games but they said it cannot be done and the easiest way is to sell your whole account for RL money.
good luck tho
Maybe read what OP said.
"selling my alpha rewards"
Alpha rewards are afaik still themost valuable items in the game. I think they ranged from 50-250€ a year ago, soif he has a full set of alpha items, he should be able to get some actual good value out of them if he can find soemthing where he can sell them.
Its illegal according to TOS of RL/Psyonix to do that.
I know there is a site where you can sell Rocket League items, but the prices are kinda low and I dont know if that is legal and/or worth it.
I personally wouldnt do any of that, but maybe if you have/find friends that are interested in the items you can exchange them for csgo skins or steamgifts or whatever IF YOU TRUST THEM OR THEY GO FIRST.