trades: add biden term trade

src/content/trades/biden-finishes-term.mdx

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+---
+summary: "Purchase hideously underpriced Polymarket swaps on the event that Biden finishes his term."
+instruments:
+    - namespace: "Polymarket"
+      ticker: "will-biden-complete-his-term-as-president/will-biden-complete-his-term-as-president"
+      name: "Polymarket Contract: Will Biden finish his term?"
+entry: 2024-11-21T05:20:41Z
+exit: 2025-01-20T00:00:00Z
+spent: 18.29
+received: 19.76
+---
+
+## Rationale
+
+Just a couple months before the end of Biden's term, Polymarket priced "Yes" shares on this market at just 92¢.
+This immediately struck me as an insanely low price.
+
+And indeed, doing some quick math,
+the probability of Biden finishing his term would have to be lower than 91.6%
+to (risk-indifferently) match the risk-free rate of ~4.4% at the time.
+**At a more reasonable forecast of 99% that Biden finishes his term,
+the expected return is a staggering 71.5% per annum return.**
+
+As far as I can tell this is just a case of Polymarket being extremely inefficient.
+Perhaps all the Trumpers are convinced Biden will resign to make Kamala president
+or something similarly ridiculous.
+
+## Execution
+
+I purchased 20 shares at an average price of 91.6¢, for a total of $18.28 before gas fees.
+I think my entry was fine although I probably could have stood to
+spend some more time and effort on my bet sizing.
+This was an extremely good price which I mostly didn't take advantage of,
+and in the future I should at least spend some time estimating what the [Kelly bet](https://en.wikipedia.org/wiki/Kelly_criterion) would be.
+On the other hand, I don't want to go overboard and end up essentially just gambling.
+
+My exit was similarly not great.
+I set a sell limit order for half my shares at 98¢,
+but on reflection I realize that there was no principled reason for this:
+I just wanted to lock in some profit to avoid the
+emotional risk of failure by "losing my whole Polymarket portfolio on a dumb bet,"
+but rationally speaking this trade was such a small fraction of my portfolio that
+I should have been almost entirely risk-indifferent and therefore just held.
+
+## Retrospect
+
+Overall I think this was a very good trade.
+As I mentioned, I definitely could have done better on my execution and bet sizing
+so I ultimately made a negligible amount of money,
+but as with most trades I post here,
+my primary goal here is to learn and improve my process,
+which I think I did.
+
+Of course, there is also the question of how predictive trades like these are
+of future success in trades with much more money on the line.
+
+It's also surprising to me just how underpriced this contract was.
+This very much feels to me like a case of Polymarket users being irrational,
+but its also possible I'm just missing something and got lucky.
+I'll definitely be on the lookout for more opportunities like this in the future.