Yes, buying Glass Flasks is worth your effort from collecting Silver Coins, as it will save up your time crafting both Glass and the flasks at your crafting stations, which can be somewhat repetitive. Also, coins can come by easier in the long run, and if you start saving up as soon as you get to loot several world chests and find what you need to craft them, you'll feel safer investing in trading them with merchants that sell helpful items at a low price. Vennor Skinflint (NPC located at Sepermeru City (Exiled Lands) and in the Camp of the Castaways (Isle of Siptah)) will sell you 10 Glass Flasks for only one Silver Coin at a time.