Selling Second: How Pre-Executing a Large Holder's Sale Changes Exit and Position-Sizing Decisions on Solana

Swap quotes and seller-side estimates price a sale as if it executes first. For holders of Solana tokens with concentrated supply, the loss that matters is the one they take when a large holder sells first, and no quote reports it. We obtain that loss by execution: at one slot we copy the accounts and programs the relevant routes use into an in-process Solana virtual machine, execute the large holder's sale and then the position's sale, and compare the proceeds with the position sold alone. We then use it to set two decisions, the largest entry whose loss behind the sale stays within a budget and the probability of the sale above which selling first is better. Across 88 actively traded tokens on October 13, 2026 (810 executions), the loss the quote omits had a median of 23% of a $500 position's quoted proceeds behind a sale of 1% of supply and 51% behind the largest holder's balance. We prove that it can never be smaller than the large holder's own average price impact; in the executions it was a median 1.8 times that impact. Under a $50 loss budget the admissible entry differed 4.9-fold between the 10th and 90th percentile of tokens and was barely related to round-trip trading cost. These effects are derived from executed scenarios at one slot; we do not observe investors' decisions or realized sales.