If possible, see if you can grab one, or even better, two pieces of your Baltic from home, sand it down and coat it with whatever your preferred coating is going to be, then take the samples to your secondhand timber dealer and have him hunt out a match for you.
Give him some of your preferred coating so he can coat a couple of his boards, thus providing you with a perfect match.
After you're satisfied that what he is selling you is the go, have him sign both your samples and his samples and take them home. After the floor is installed, sanded and finished and it all looks the goods, well, obviously throw the samples to the chithouse. But, if after the install and finishing is done and the boards don't look anything like both lots of samples, you will still have a leg to stand on if you feel the need to discuss it with the secondhand dealer.
The reason I'd go to theses lengths is seeing as you are a novice it's very easy to buy the wrong material, as Baltic Pine comes in three different colours and it's tough to know the difference when looking at racks of old material in a wood yard.
And at $20 bucks a LM I think it's only fair you make them earn their dough.![]()