Professional chemists absolutely DO NOT buy stuff on ebay or amazon. The reason is very simple: You just don't know if it's pure. Testing that takes a lot of time and money.
Let's look at one example: I want to buy something that costs either 500 bucks as reagent grade from Sigma or 50 bucks as unknown grade on amazon. With the usual Sigma discounts it's going to cost the uni or company 300ish. But you get it the next day, it's pure and if for whatever reason it isn't, you get a free replacement the next day.
Or i get the amazon variant and have to run at least XRD and EDS for solids and NMR plus some sort of MS for liquids. The cost of running those machines alone is higher than any potential savings, and we haven't even looked at the cost of buying those machines or even paying the chemist who has to do it. If it costs your company between 50 and 100 bucks per hour to keep a chemist you think twice before you tell him to waste a day analysing something from amazon.
You either buy it from a chem supplier, if that's not an option (if it costs more than like 1-5k) you buy the pure raw materials and do it yourself.