Large prime numbers for sale

You can use a Baillie-PSW test to efficiently test numbers for probable primality. There are currently no known counterexamples for this algorithm, so it gives a modest degree of certainty.

However, Pari-GP has an APR-CL implementation, or there are a number of ECPP implementations available that can check it.

It doesn't take all that long to actually prove primality of a number this size. It takes almost no time at all to demonstrate they are probably prime, as this person appears to be doing.

Now if I got a primality certificate (which I could check myself), 1 Euro might not be such a bad price.

The cost scaling seems a bit off balance.