Have never purchased it before, this is my first time in the system.
There are several factors that determine availability of any commodity.
System State has a large effect on availability as well as customer
Demand (or
quantity demanded for the business nerds). It is possible for a rush of customers to deplete a station's supply of a commodity, such as during Community Goal events.
There are also two "maximums" when trading. There is a "
transaction" maximum which just like it sounds is the maximum amount you can purchase in a
single transaction, then there is what I call a "
ship" maximum which is the maximum amount a station will sell you in total before you have to go and get rid of some by selling it, donating it, jettisoning it, whatever.
A CMDR can usually purchase only a few units of a commodity in a single transaction, "usually" because
sometimes a station will sell the ship-maximum all in a single transaction.
A CMDR can purchase the transaction-maximum, then leave the commodities market, come back in (over) ten minutes and the station will sell that same transaction-maximum over and over, every ten or eleven minutes until the CMDR reaches their "ship" maximum and the CMDR must go and dispose of some. You can usually tell you have reached your ship-maximum when you have been buying your transaction-maximum several times every 10 minutes and suddenly the amount the station is willing to sell to you drops i.e.: you have been buying 12 of something over and over and suddenly the station will only sell you 8. You have reached your ship-maximum. Go sell it, donate it, whatever and return for more.