These kind of scams are everywhere on Amazon, not just in brewing. They rely on catching :
a)people who don't read carefully and assume that since it's on Amazon it will be a fair price
b)shopping bots who just automatically buy the "cheapest" available on Amazon without thinking what a fair price would look like, when other places are out of stock - so it works better with the more obscure stuff like BE-134 that may not be in stock in other places
c)really rich people who see the price and just don't care
No they won't sell very many - but they don't care. Say a "normal" retailer is selling yeast at £5/pack but it costs £3 to buy and takes 10 minutes of staff time @£10/hour = £1.67 to process each order (from ordering from the manufacturer/wholesaler, unpacking the bulk stock, packing it up and labelling it to send to the customer and all the overheads of sorting out VAT returns etc). So 33p profit per pack of yeast.
Or you could sell 1 pack at £35 and make £30.33, as much profit as from selling 91 packs at normal price - and with a whole lot less hassle.