It doesn't matter what season it is when you buy the timber. The decisive factor for quality is when it was felled (preferably in winter), where it grew (preferably in cold regions where the growth rate is low and the timber is therefore denser than where it grows faster in warmer regions. For example, there is a big difference between Småländsk pine and norrlandsfur, the former being vresved, the latter straight-grown), how it is sawn, how it is stored after sawing/planing, how it is transported, how it is stored at the point of sale, and finally how it is stored at the construction site before it is built in and even after.
Black spots (in the worst case black mold, in the best case just a little splash of lubricating oil from the saw during sawing, take a sample and let it be lab-analyzed) that are coated with boracol (the chemical boron in liquid form) usually do not cause problems afterward. Hardly before, either, if the timber is given the right conditions for mold not to grow. But if you still want greater security against further spread, it can be arranged by drilling in boron cartridges (the chemical boron in solid form) into the timber in a few places.
If you have the mold analyzed and can prove that it was there before your handling, I think you can contact the supplier and request compensation for your additional costs + reduction of the price. The alternative is that the supplier gets the timber back with a surcharge for your transport, etc.
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Byggaren
