To estimate a maximum purchase price for the coin in question, let’s consider the following factors:

1. Current starting price: 9000 euros.
2. Coin condition: 90 (EBC), indicating good preservation.
3. Auction history: It has been auctioned once in the past with a starting price of 5000 euros and an estimated price of 7000 euros, although it did not sell.
4. Average starting price: 7000 euros, suggesting that the market has valued similar coins in that range.
5. Number of times it has been auctioned in the past: Only once, which may indicate that there is little information about its demand.

Given that the current starting price is significantly higher than the average starting price and the estimated price from the previous auction, this could indicate that the seller has high expectations. However, the fact that the coin did not sell in the previous auction suggests that the market may not be willing to pay that much.

Taking all these factors into account, an estimated maximum purchase price could be set in a range between the average starting price and the current starting price. A reasonable price could be:

Estimated maximum purchase price: 8000 euros.

This price reflects good preservation and the expectation that the coin could attract interest, but it also considers the lack of previous sales and the average market price.