Well, we don't know what your conversion rate is now, so we can't advise how you might be able to increase it.
Mine usually hovers around 3-3.5% , and, bearing in mind the sort of items I sell, I'm not sure there is much I can do to improve that. It's one of the reasons I didn't go further with pay per click. If only one in thirty viewers are buying, it's not going to be viable to pay extra for page views.
I would imagine it's much the same with stamps. Dozens (hundreds?) of other sellers are selling similar items to yours, and there is basically little difference in either the quality of the item, or the price (with some exceptions).
It's also important to note that not all visitors to a page are potential buyers to begin with - and you will never convert these individuals into buyers. They may be competitors comparing prices, people trying to price their own collections, or general "window-shoppers".
The only thing I do sometimes, which may help, is to look after my exisiting customers. Once they've bought a couple of times I send them a thank you note, with a coupon giving them 10% off anything in my shop for three months; if they continue to buy, I renew the coupon.
