To estimate a maximum purchase price for the coin in question, we can consider several factors based on the provided data:

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Current Listing Price and Previous Prices: The current listing price is 350 euros, which is significantly lower than the average listing price of 670.02 euros and the average auction price of 908.56 euros. This suggests that the listing price may be underestimated.

3. Auction History: In the past, the coin has had auction prices that varied considerably. For example, the highest recorded auction price is 450 euros (in 2013), but there is also an auction price of 2805 euros from a 2015 auction, although the latter seems to be an exceptional case.

4. Sales Trends: Out of the 7 times it has been auctioned, it has sold 5 times, indicating a consistent interest in the coin. This may influence demand and, therefore, the price.

5. Comparison with Average Prices: Given that the average auction price is 908.56 euros, and considering the current condition of the coin, an auction price that approaches this average could be reasonable.

Taking all these factors into account, an estimated maximum purchase price could be between 700 and 900 euros, considering that the condition is good and that interest in the coin appears to be solid. A price of 800 euros could be a reasonable target, as it falls in the middle of this range and aligns with the average auction price.