Some people do this yes, also buy like every single item to have the monopoly and define a higher price.
I don't do like this because the main goal is to always have some supply in the bank (the latest items : volatiles, ores, gems, crystals, plants...). So I do my daily scans, if I find some of those below the average price (Auctioneer addon : get it, it's good to know the ongoing price of every item you get, like minipets for example) of the moment I buy it and stock it that way it's available for the guild, but when somethign is in shortage I sell most of it for the full price or even more, that way I'll have the money to buy more when it will be cheap again. This takes a lot of time, but the more money you win the more you can invest, and your guildmates will always have everything they want for their profession.
I sometimes resell some old items going for a good price (depends on the server, but here cobalt and saronite demands is high and supply is low).
It takes like 20 minutes a day, I know some people are way more involved in this (of course on myserver I can't compete with them so I just ignore the items they focus on, in my case glyphs).
Please don't do this on my server