Knives from the 2017 Halloween Box are no longer obtainable, so trading is the only way to get the Phantom now. It's a common-rarity knife, not a chroma, and it never had much of a value history to speak of. People who want it usually go after it for the event origin and the Halloween look, not for profit.
Phantom (2017) is a Common knife in Murder Mystery 2. Its combined value is 10.5, #38 of 335 common items and #339 of 921 valued items overall. Demand is low: it sits in inventories for a while, and buyers often ask for a discount.
Closest in value: Combat II (10.5), Sparkle4 (10.5), Arctic (2022) (11). Use them as a benchmark for a fair one-for-one trade.
Value history
What it is worth in other items
How many of each item equal one Phantom (2017) by value. Demand differs between items, so check it before you trade.