That's high? Holy cow, . . ., I'm just going by the Mtgox exchange rate
I think the current MtGox exchange rate is somewhere around 0.09248 BTB per USD, so if you sell for 1 bitcoin, then you are looking at a value of approximately $10.81 for your volume. I don't know if this is high, but you were the one who said that manga is "usually $3 - $5 per volume."
. . . what happens if someone decides to sell a ton of bitcoins and then the price drops massively? . . .
As best I can tell, it seems that the exchange rate has been somewhere between 0.07704 BTC and 0.10267 BTC per USD for the past month.
You said that manga is usually between $3 and $5 per volume. Lets assume you want to receive approximately the equivalent of $4 which would leave some room for exchange rate fluctuation within the "usual" price of manga. $4 * 0.09248 is approximately 0.3699 BTC. If you adjusted your prices monthly, (and kept an eye on the exchange rate every few days to adjust for unexpected large swings in exchange rate), then a price of 0.37 BTC would have gotten you somewhere between $4.80 and $3.60 on any given day in the past month. I'd say that all those prices fall within your $3 to $5 range, so your customers aren't getting ripped off.
If there is a large sudden change in exchange rate, you could end up with a price that either offers your manga for REALLY CHEAP, or REALLY EXPENSIVE. If it becomes really expensive, it seems unlikely that your customers would get "ripped off". They would most likely notice that your manga costs more than they want to pay, and they'd simply not buy it until you adjust your price downwards.
The real risk you run is if the exchange rate suddenly makes your manga really cheap. In this case you have a few easy options. The first and easiest is to accept the risk under your own personal belief that the exchange rate will eventually recover and just hold on to your bitcoins until it does. The sudden "cheap" price might encourage individuals who hadn't quite made up their mind to make the purchase.
How do you plan to handle price listing and payment processing? Are you creating a website where purchase of your manga will be automated? Will they email you with a request to purchase? Will you simply post a donation address? Will they contact you via phone? Your other options for quickly adjusting to sudden changes in exchange rate will depend a lot on the systems of transaction coordination you are planning on using.