There are many reasons why a dealer may decline to buy your coins. It's important to keep in mind most of these reasons probably have nothing to do with the coins or your asking price. What follows are some of the reasons.
Some would like to think of the business side of our hobby as an industry. Actually, there are only a few coin factories and many dealers who must diligently conserve their working capital and carefully consider the wisdom of every inventory purchase. There may be times when the dealer you approach simply decides investing in the coins you have for sale isn't a wise thing to do. The next dealer may be delighted to acquire what you have for sale.
Sometimes coin collectors seem to forget a basic rule. Any merchant is usually best acquainted with the type of product they are selling. They are usually most interested in buying these same kinds of things. This is why wholesale paint salesmen don't bother to hawk their wares to the owners of a ladies dress shop. To expect that every coin dealer is equally prepared to make a fair offer for your rare Canadian pattern coins and your bullion related material is simply expecting too much.
Most coin dealers I know will buy anything if they think the price is right. What I may think is a reasonable price to pay for something may only be a fraction of its fair market value. Previous experience has taught me to avoid going in the opposite direction. Easily the biggest mistake the average coin seller makes is to offer coins to unqualified buyers.
Using myself as an example, if someone were to offer me a nice selection of early Canadian currency I certainly wouldn't pass up the opportunity to buy it. Unfortunately, I've handled so few of these items I'm not well acquainted with market values and have no customers waiting for such things. It can be difficult for a coin dealer to admit they don't know enough to make a fair offer. You should respect the dealer who declines to make a purchase for this reason.
Level of Business Activity
This can work in both directions. If I were to call a major bullion dealer and offer them a single gold maple leaf they might express no interest in doing the transaction because their business is geared to handling larger buy and sell orders. If I offered the same coin to the owner of a coin shop they might be pleased to add a single piece to stock. It can be a huge mistake to assume the “big guys” pay the most or a small business is that way because the owner doesn't pay competitive prices.
None of The Above
Most sellers of coins adopt optimistic expectations. This includes both dealers and collectors. I can't begin to count the times I knew a certain coin was easily worth X and finally managed to sell it for X less 20%. There are six words everyone who buys coins should remember. “Buying is easy. Selling is hard.”