Although the English-language documents in the Files section describe rules reasonably well, they lack some precision regarding the purchasing of tiles.
The printed game rules are imprecise on this point, leaving players to come up with their own "common sense" procedure for purchasing tiles.
User Ajax's "Quick Reference" document is clearer than the game rules, identifying "Buy Tiles" as a phase distinct from and preceding the "Play Tiles" phase. This simplifies things, although it doesn't accord with the printed game rules section stating tiles can be purchased even after an opponent has selected a tile to play.
(The only fault I find with Ajax's description is that if multiple players wish to purchase tiles, they're to resolve their purchases "in order of asking". This unfortunately can lead to something of an undesirable verbal "race" (in certain situations) to be the first purchaser after a round ends.)
I'd suggest it's better to allow players to announce their purchase decisions in any order and without time pressure before the "Play Tiles" phase, and then to resolve those tile purchases by point value.
At any rate the issue can be important, as the purchase of tiles is accompanied by the movement of player pawns. And whether one player moves before or after another can introduce a difference in the game's progression.