The Card Trader provides some core cards that you need for building Hazys and other oddball decks. There are also a handful of general purpose PvE cards that you might find yourself needing.
Here are some notable cards you should save your gold and stones for, sorted by usage:
Core Hazy beater alongside Peryton. Known for their untargetability, swarming, and OTK potential.
Life Cost 0's ace monster. Usually run in tandem with Solemn Scolding/Kuribohs to control your LP.
F2P option for Tribute heavy decks like Hazys.
Enables 30 card decks to have scary utility. Great for decks that use resources from the Graveyard such as Gem Knights, Bujins, or Spellbooks.
Quick-play Spells are in season. Counters notable cards such as ECon, Forbidden Chalice, Mask Change, and Mayhem Fur Hire.
Tech against Warrior-heavy archetypes like Amazons and Masked Heroes.
Unique addition to Masked Hero decks that walls stronger boss monsters.
Archfiend staples, prevents all Quick-Play Spells and banishing respectively.
Tech for SSA decks, allows summoning of cards like The Legendary Fisherman II without Tributing.
If you have the spare 'R' jewels, consider making this card prismatic for a bump in Duel Assessment Points.
Extra Fusion subs to replace Yugi/Kaiba/Joey's level-up rewards.
Switcheroo but in card form. Used to re-roll for specific pieces such as Fusion materials/Poly.
Age-old deck thinner used in turbo decks like Elements United!
Easily fused from Thunder Dragon, usually to help boost Vassal above 9999 ATK
More draw power designed for Spellcasters; works especially well with Gravekeeper's Recruiter or Blue Dragon Summoner.
Turbo option against LDs or event duelists that run backrow destruction.
Part of a strong auto-dueling deck, or for Water-centric decks in general.
Counters specific LD cards such as Kaiba's ECon.