I feel this is more "limits of PredictIt" rather than "limits of prediction markets". Doing the equivalent analysis for Betfair for someone in the UK. (2% charge on profits, no fee for withdrawing). Right now the market for Trump to win is (£1200) 2.78 / 2.80 (£7421) Those imply probabilities of 35.97% and 35.71% After fees, 36.44% and 35.00%. That's not a bad spread, and you can do this in much more size. (If you're willing to work a bit, you can comfortably get on £10,000s at prevailing levels).