Most of those do sound a bit awkward, and also like you're trying to cash in on the popularity of a site that isn't even popular anymore.
That said, product names are usually less important than people think. It's your baby, you want it to have a cool name, but if you're spending hours agonizing over the name, that's time that could be better spent on improving the product. If it's a great site, people will flock to it regardless of the name. Just look at google - you don't even notice anymore that it's a silly name, do you?
So if your product name sounds great with a "ster" at the end and the domain's available for cheap, just go for it and go back to making it a site so great people will love it by any name.
Of course that would depend on what the first part is!
You can't just say it's old because it ends with
pwnzster.com don't look too old right?
'-ster' does seem a bit old, as I suspect '-ly' domains will in another few years.
They're not 90s, Netflix just announced Qwikster, I had a domain name earlier called loopster but not anymore. Ending with ster can sound catchy sometimes. Nothing is 90s or 80s as long as its catchy and easy to remember.