If you think you have one of these rare holographic, gold star, or base set cards, then the first step to take before even thinking about selling them is getting the cards professionally graded
Theoden is certainly the former, but he is unfortunately not the latter
Friends in Paldea 137 This textured Full Art Supporter is a prime example of high print volume making a rare card affordable
Queen Marchesa is a great intro to cards that use the monarch mechanic to their advantage
During the opponent's Turn when a Monster declares an Attack, "The Hidden City" can flip a face-down "Subterror" Monster to not just active their effects, but negate the Attack as well
Venasaur EX SAR is selling for around $75, or closer to $350 should you be lucky enough to get it graded a 10