sadly , sounds the same as if you sold 70 litecoin for the wrong price and someone snatched it up.
It's not quite like that, is it? If you sell LTC at a proper exchange that has decent market depth and proper order matching the worst thing that could happen is that you would sell it at the highest bid price, but not at a 90% or 99% discount as happened with some hashlet sales. But that is not the model Zen chose for its marketplace, i.e. there are no bids, just "for sale" listings. It's more comparable to e.g. eBay or craigslist than to a currency exchange. However on eBay or craigslist the seller has more time to decide to back out of the transaction - perhaps at the expense of bad feedback but still. Zen marketplace seems rushed and poorly designed for its intended use.
The difference is that there are different sizes and types of hashlets and the markets is not deep enough so that this would be possible. Also the differences between the types of hashlets is small enough so that most people will not case what kind they get, only the price and the hashrate, however the types do make a big difference to gaw, so this is why they have the market setup the way they do.
If gaw were to allow the OP to cancel his sale after the offer was filled then the buyer would have an issue with gaw because they would not be able to take advantage of a good deal they found and paid for
An exchange-type model would be driven by supply and demand, if there is no demand AT ALL the seller wouldn't be able to sell, but the OP sold hashlets (albeit at a very very low price), so there is demand. Making buyer bids available wouldn't make it worse than it is now, and in cases where there is demand it would make it much better for both sides. Exchanges routinely deal with hundreds or thousands of different assets, a few types of hashlets shouldn't be a problem. However if GAW prefers to stick with the "marketplace" model and not "exchange" - see below.
Price errors are not "good deals". Yes, it's the sellers fault. No, it's not wrong to allow the seller to cancel the sale in the current Zen marketplace model, which is more like eBay, Amazon, craigslist, or your local flea market, ALL of which allow that. Yes, the buyer would be unhappy. There are ways to deal with that too, if GAW wanted to make this a civilized marketplace. E.g. non-refundable listing or transaction fees.
I think the point I'm trying to make is that it's not necessary to invent anything new - there are proven models that work, and GAW also has the advantage to learn from mistakes of others. Instead they're choosing to make their own mistakes and punt the responsibility on users. Not cool.
There is really not enough demand so that it would make sense to have bids listed on their exchange/marketplace/whatever you want to call it. Having bids listed would result in sellers selling for much less then what their hashlets are worth and GAW would have a hard time selling directly to the customer in the first place.
I believe that the OP did have the option of canceling his order however it was filled too quickly for him to be able to cancel. Buyers have the option of sorting by price per MH so someone likely had sorted by this way, saw his order and quickly purchased.
I have also sold on gaw and it takes a lot to create a sell order, you have to click several times, so the OP was either not paying attention to what he was doing or is trying to scam GAW to give him a refund (while keeping the buy order on his other account)