Just to add to the whole purchasing of credits question - if you do a free credits offer that awards 5 or more credits, this also counts as you purchasing credits, and allows you to sell on the market, receive private sales & make secure trades.
When starting elo the main concern for new players is having enough cards to use because of the weekly format. try picking up a clan that hardly faces getting banned and is affordable.
Fang pi clang is great and cheap. you can buy the staple cards for an affordable price. Not to mention they have one of the best LD cards which is available now so you can complete the missions and not have to pay for a pricey 4 star card
rescue are cheap, roots, ulu watu(some 4-5* cards are pricey)