During a bull market rush, like that Ethereum began to experience in late May 2017, it becomes more profitable to mine a currency which gains an immense amount of value seemingly overnight. This happened before with Bitcoin and Litecoin, and is likely to happen again with the next big coin. Welcome to your first economics lesson: supply and demand.
New miners and existing miners will begin to buy up supply of popular mining hardware, making it virtually impossible to buy stock. Companies (like Micro Center and Newegg) begin to implement policies limiting the number of stock any one person can buy in an attempt to stop the products being sold to one group of people, and new miners have a difficult time getting started. Resellers will then purchase the hardware to sell on eBay, forums, and subreddit like /r/HardwareSwap. Affordable motherboards like the Biostar TB250-BTC began to sell on eBay for three times their cost.
Miners then have three options:
- Buy the hardware at the premium and continue the cycle of price-gouging
- Aggressively monitor the price and stock of hardware from first-party retailers (Newegg, Amazon, Micro Center, etc)
- Wait until the hardware price bubble pops and stock is reintroduced at a normalized price
If you still wait the cards but do not wish to pay nearly double the retail value, there are several tools at your disposal. Monitor stock with sites like NowInStock and browser plugins like VisualPing. You can also research uncommon parts to ensure they meet your needs (this mostly works for motherboards) and buy them up. During the bubble, I built a new mining rig full of RX 480 and RX 580 8GB cards and did not spend above retail price for a new card.