For me I'd still consider wether this was viable over the long term, if your making an average of £6.60 per book then by my calculations you'd need to be selling over 4500 books a year to make £30k, at the minute your selling around 900 a year (again based on your calculations), so this would equate to a five times increase in your current trading levels, is this realistic?
£30k is only an okay wage in this day and age, not what could support more than one person really.
You also have to take into account that storage costs will grow at this level as will your time requirements and other associated costs, so you wouldn't be left with £30k based on your information already supplied.
Like some others have mentioned as well as sorting out your storage issues you need to develop other ways of making an income from them, if buying cheaper is not possible and you can't raise prices then it's going to be a long difficult road for little profit.
I'm honestly not trying to be negative, but even if you had free storage costs would you be able to ramp it up to the required levels of business?
Only you would know this and you have to be realistic because if it's not possible then would you be better in looking at other ways of generating income instead?
Another thing to consider is what if Amazon cancel their Marketplace? It might not seem likely at the minute but who knows what's going to happen in a few years, Amazon is a private company and it seems like your placing your future business in their hands.
