It's called normal market forces (in the Capitalist Western Society)
Every company - BanDai included - makes money to stay in business - the more money they can make the better it is for the owners of the Company and some of that money can be re-invested to make more / different products for consumers (ppl like us who buy the products).
Every seller tries to make money and the more money they can make the better for them - and the ID and +C are the most recent (and most popular) right now - so they are the ones that can achieve the highest prices.
Being "fair" or "not fair" has nothing to do with it - it's business.
If you have a product that you can sell to someone for $70 and that someone will buy it, then why would you sell it for less?
If you are a seller who worked hard to find a good deal on a case of tamagotchis and managed to buy 50 of them for $30 each, you have just paid out $1,500 of your own money and you need to try to earn at least that much for each sale back into your savings.
You also need to work in "invisible" costs like postage and packing, how much time it takes you to pack a tama carefully so that it doesn't get damaged in the mail, the cost of the padded envelope to send it, the petrol it takes for you to drive to the Post Office and mail it, etc. etc. These all need to be worked into the price you charge a customer.
If a seller tries to charge too much, ppl won't buy from them and they lose out (and maybe have to reduce their prices);
the best thing to do is keep looking around on ebay and Amazon and all the other sites for reliable sellers who charge reasonable prices.
If they are all charging around $50 for the same kind of tama - then that price is what ppl are prepared to pay and you have to decide if you want to pay that or if you don't want the product.
That's just the way business works I'm afraid