feat: add avoiding the sell-buy trap in STRATEGY.md.jinja

Author: S1M0N38

src/balatrollm/strategies/default/STRATEGY.md.jinja

@@ -568,6 +568,25 @@ Once you identify a strong Joker or receive relevant Planet cards, commit fully:
 
 Money management determines your long-term power growth. Balance immediate needs against compound benefits.
 
+### Avoiding the Sell-Buy Trap
+
+One of the most common economic mistakes is frequently selling Jokers to buy new ones. Unless a Joker has special sell value properties (like Egg gaining $3 sell value per round), **all items sell for exactly half their purchase price**. This creates a devastating economic spiral:
+
+- Buy a $6 Joker → Sells for $3 → 50% loss
+- Buy an $8 Joker → Sells for $4 → 50% loss
+- Repeatedly cycling Jokers burns money twice as fast as keeping them
+
+**The Math**: If you buy and sell 3 Jokers costing $6 each across three shops, you've spent $18 but recovered only $9 - a $9 net loss. That's equivalent to throwing away nearly two rounds of maximum interest earnings.
+
+**Better Approach**:
+- Only sell Jokers when they actively conflict with your strategy or a vastly superior option appears
+- Treat Joker purchases as permanent investments, not experiments
+- If unsure between Jokers, skip both rather than buying one to "try it out"
+- Reserve selling for special cases: Egg at high value, Jokers that have fulfilled their purpose (like early economy Jokers when transitioning to late game), or when a game-winning Legendary appears
+- Remember: A mediocre Joker you keep is often better than losing half your money to swap it for a slightly better one
+
+**Exception**: Jokers with growing sell value (Egg, Gift Card) or those providing immediate one-time benefits can be sold strategically once their purpose is served.
+
 ### Interest Optimization
 
 - Reach $25 as quickly as possible to earn maximum $5 interest per round