@OP: Get ready because there's more of these types of "rare goods" engineer bribes. Stations have a "per customer" limit on rare goods where if you have a certain amount already in your hold, they won't sell you anymore. The allocations are also on a timer (I think 10 minutes), so even if your hold is empty, you won't be able to get the full limit at once.
In practice this usually means you have to make a bunch of trips back and forth between the engineer and the station selling the rare good. However if you're really determined to beat the system, there are two workarounds. One is insane, the other is super-insane.
The less insane method: wing up with other CMDRs. You wait outside the station, they go into the market and buy their max allotment of goods, then fly out to you and dump them. You scoop them up and they fly their empty cargo holds back into the station to buy more. Repeat as necessary. Because of the refresh timer on rare goods, the more CMDRs the better.
Super insane solo method: Equip a good limpet controller and lots of limpets. Go buy your max allotment of goods. Fly a bit outside of the station and wait 10 minutes. Dump all your cargo. Quickly fly back into the station with your empty hold, and buy one more allotment of goods, then quickly fly back out of the station and pick up the cargo that you dumped outside. Dumped cargo canisters are on a self destruct timer, so you have a short amount of time to get in, get out, and pick them up again. Wait another 10 minutes for the market to refresh, and then dump all your cargo *again* and repeat the whole process. Keep repeating until you have the desired amount of cargo in your hold, and then congratulate yourself on your ingenuity.