Matching recycled boards to existing Baltic pine

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.

hi - if you must match then take time to look for boards that are definitely the exact same profile. They will be around, it just might take a bit of time to find them. As far as matching colour and patination, it is possible but most unlikely that you will be able to. What a number of my clients have done is replace boards from a smaller room in the home, then use the 'new' ones in that room. It is a lot of messing around though and if you're anything like me, you want to get on with other parts of your renovation work.

If you're willing to sand all the boards - the new ones and those leading up to the area to be refloored - you'll achieve a good match. If you want to get as good a match as possible to the existing, without messing with them then it is hard to advise. You can remove several boards from the existing floor and mix-n-match them all together to help the transition, but you run the real risk of damaging the existing boards when removing them.

Maybe it would be best to just try to find boards that are already a reasonable match

Let me know if you need referrals .... also please state how much (lineal metres) you need to do the job.