This market resolves based on candidate with the highest number of electoral college votes after all votes are certified by congress.
The options are mutually exclusive; only one will resolve to yes.
"Winner Electoral Votes < 270" resolves to yes if the person with the most votes for POTUS receives fewer than 270 votes (e.g. results that lead to a 12th amendment contingency).
"270 <= Winner Electoral Votes < 280" resolves to yes if the person inaugurated POTUS receives between 270 and 279 votes.
...
"325 <= Winner Electoral Votes" resolves to yes if the person inaugurated POTUS receives 325 or more votes.
The market resolves based on the congressional certified electoral votes. As reported by the national archives (2016, 2020) or several reliable news sites.
Edge cases:
Results are based on what congress certifies. This means faithless electors or rejected states may affect the total.
In 2016 there were two faithless electors for Trump. So this market would have resolved to the range including 304.
If congress certified there is no majority (e.g. a third-party candidate wins some votes) and the election is resolved via the 12th amendment contingency the market still resolves based on the certified electoral votes (likely "Winner Electoral Votes <= 269 ").
This market resolves based on the results certified by congress. So if the winner fails to be inaugurated for any reason that does not affect the market.
The US ceases to exist or undergoes a drastic change to the electoral process: Resolves N/A.