All the above is good advice, especially Graham's and Woody's.
I would expect the average "margin" or "mark up" in the retail angling business to be in the region of 40% to 50%. But it's not that simple!
To make it work, many retailers will use special offers, linked "packaged" purchases, other services and similar incentives to gain customers. Larger ticket items are more often than not sold giving a fixed (small) amount of margin rather than a percentage. Those trades are then often packaged with other services (pole kits, elastication etc etc).
Unless you have loads of spare cash lying around, you will need some financing to purchase stock. That financing will be expensive and therefore will not financially allow you to get the big ticket stock in and sell it in that way (the financing will cost more than the fixed margin = loss!).
Large retailers may well have special arrangements with the major manufacturers. Special financial packages or more commonly trading direct where the customer buys from the retailer but the "stock" is held by the manufacturer who delivers direct to the customer (the stock never touches the retailer).
Getting finance means drafting a carefully prepared Business Plan to present to your ever friendly bank manager. They will help in that preparation but a Start Up Retail Business Plan is a wonderous work of art and needs huge amounts of preparation and local research to be effective (and therefore stand a chance a gaining economic finance).
Good luck!!