As I understand it's cheaper to buy at the station in Warsaw than online.
Actually not, because when buying online you can catch discounted tickets, which definitely won't be available at the station before departure.
If I buy it on german train site, it will be around 80 Euro for 2nd class ticket.
Well, full fare standard ticket for BWE (Berlin-Warsaw Express) EuroCity train cost 48.6 EUR one way (including seat reservation) or twice for return. Or even cheaper in case you'll use evening EuroNight train. While available online discounted tickets (Europa-Spezial Polen) for August 6 start from 39 EUR per person.
Will it be significantly less if I buy it in Warsaw?
In case of international route and tickets, standard fare is almost identical no matter in which country (Poland or Germany) you purchae, so in case of full fare standard tickets no big difference here. Main difference comes from offers, which might be offered only by one railways company etc.
Can I rely that there will be seats available?
Usually those EuroCity isn't that busy, unless it's holidays or any other peak period. Anyway, it's easier and cheaper just to buy now discounted ticket on DB website, thus guaranteeing your seat and better fare.
Also it's possible to travel even cheaper than 39 EUR by using polish domestic tariff till the border, but in such case you'll have to make changes and it will take a bit longer.
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