In a theoretical hypernet offer of a 5,000,000 isk item with 10 tickets for 1,000,000 isk each:
The expected value of a ticket is the value of the item you’ll win multiplied by your chance of winning that item.
There are 10 tickets, so the odds of a ticket winning is 1 in 10, or 10%
10% times the prize value of 5,000,000 isk is 500,000 isk
The expected value (500,000) less the cost (1,000,000) is a 500,000 isk debt.
Buying your own ticket doesn’t really change this formula.
Someone else buys your ticket: You get 1,000,000 isk, you lose an expected value of 500,000 isk (=500,000 isk for the hypernet seller on average)
You buy your own ticket: You lose 1,000,000 isk, you gain your 1,000,000 isk, and an expected value of 500,000 isk on your ticket for the item you also paid 500,000 isk for (= 0 isk for the hypernet seller on average.)
The seller reduces their potential to lose their item being sold by the same proportion they reduce their profit potential.