You must realise that SQL server was not a M$ product to begin with, it was a well built database server before M$ bought it.
A company as large as Ebay will most certainly not be using mySQL. They’d be using either SQL server or Oracle. Oracle being run on Unix type systems.
Because php and mysql are so well known to be a good pair, you might get the idea that php works only with mysql. Not true. php works with SQL server, Oracle, PostGre, etc…
If you are going into ASP, don’t go asp3. go .NET. ASP3 is not under development and hasn’t been for several years. They make a few changes to VBScript every once and a while, but that is about it. M$ is all .NET now. If you do go .NET I would probably choose SQL server over any of them.
But of all the databases out there, Oracle is probably the best. But it’s licensing is not cheap. SQL Server will be a lot cheaper and SQL server is really quite mature. It has way more features than mySQL.
Read the mySQL licensing. It’s not cheap and under certain circumstances the licensing requires you to pay, mostly I think when it’s wrapped up with commercial applications, but I don’t know all the details.
It really depends on how heavy a load your db will have to handle. There are a lot of e-commerce sites running mySQL and are running fine. It is hard to say because I don’t know anything about your company, what you sell, what your projected audience will be, in numbers… etc…
Personally I like MYSQL since its a more fliexible and runs on linux (a more redunant OS for hosting).
Redundancy has nothing to do with OS and although Apache - Linux / Unix is a good portion of the hosting market, with a good admin a M$ server running iis and SQL can be rock solid and there are a lot of M$ servers out there serving you web pages. I personally prefer Linux / Unix servers, but it’s really a matter of taste.