To estimate a maximum purchase price for the coin in question, we will consider several factors:

1. Starting Price: 15 euros.
2. Condition: The coin has a condition rating of 75 (MBC), indicating that it is in quite good shape.
3. Auction History:
- It has been auctioned 4 times in the past, of which it has sold 3 times.
- The average hammer price in previous auctions is 12.33 euros.
- The starting prices in previous auctions have been quite low, with a maximum of 10 euros and a minimum of 1 euro.

Given that the condition of the coin is quite good (75), and considering that the starting price is 15 euros, which is higher than the previous average hammer price, we can estimate a maximum purchase price.

A reasonable approach would be to consider an increase over the average hammer price, given that the condition is better than the coins auctioned previously. We could apply an adjustment factor based on the condition and market interest.

Estimation:
- Previous average hammer price: 12.33 euros.
- Adjustment for condition (considering that 75 is quite a good state): we could consider an increase of 20-30% over the average hammer price.

Calculating a 25% increase on 12.33 euros:
- 12.33 * 1.25 = 15.41 euros.

Therefore, an estimated maximum purchase price for this coin could be around 15-16 euros. This aligns with the starting price, suggesting that it could be a reasonable price for an interested buyer.