Comparison site for EU SIM cards?

There are several concerns besides "cheapest". An ideal EU travel SIM must satisfy the following properties:

1. Available for purchase on Ebay or some other website that ships internationally
2. Can be activated from abroad or can be purchased pre-activated
3. Its possible to top up the SIM with a foreign credit card without huge surcharges. I.e. Czech phone operators block foreign credit cards on their website and you have to pay 10-20% to resellers.

To narrow down the search, an optimal strategy is to search for "activated"+"EU SIM" on Ebay, giving us a list of SIM cards that could be easily purchased in practice. As of today, here are the offers I'm seeing:

1. Lebara/UK. 15 GBP for 9.4 gigs of data within the EU, as per their FUP document. Comes out to 1.85 EUR/GB.

2. Three/UK. 15 GBP for 20 gigs of EU or UK data, as per their FUP rules. Comes out to 0.75 EUR/GB.

3. Labas/Lithuania. 26 EUR for 3 gigs of EU data. Comes out to 8.7 EUR/GB.

4. NOS/Portugal. 29 EUR for 10 gigs of data. Comes out to 2.9 EUR/GB. No special limits for EU roaming.

So currently the best pre-activated SIMs to stock up on are from Three/UK. This listing is a good example of what seems like a great deal.

This StackExchange post will inevitably become outdated in the future, so one's best strategy would be to head back on Ebay and check the latest offers on hand. I can't find a website actively tracking such SIMs at the moment, but at the end of the day knowing that some operator in Cyprus or Luxembourg has a great EU roaming deal is pointless if you don't plan to travel there and don't have a way to ship their SIMs to another country. So Ebay is probably the best resource for these kinds of deals.