There is a lot of equipment and supplies available on line, but you must factor in shipping. There may or may not be good stores near you. Often there are stores SOMEwhere, but it means a longer trip. Obviously, shopping around by phone, then planning a trip that covers several stores, and knocks out a big part of the shopping list would play a big part in finding reasonable prices.
Acronym for this: FAFS= Far Away Fish Store: So good it is worth the trip.
I have purchased things from Drs Foster and Smith, Big Als, and have found both places to have reasonable pricing and prompt shipping. I have bought things as big as a canister filter from them. (Don't remember which). Shopping at Amazon or E-Bay may be more of a risk, but if you know the make and model number, and what it is supposed to come with, you might find some really good deals.
For fish food: www.almostnaturaltropicalfishfood.com
Best prices for most foods, fast shipping, nice guy to do business with.
For fertilizer: www,aquariumfertilizer.com
For heavier things, though, I shop locally.
Around here used products are half or less of their original price. Depends on the quality. If you are looking at used tanks, make sure the seller is willing to fill it so you can see that it does not leak. Electrical things (filter, power head, heater, light) should be proven to work. Lights will be assumed to have old bulbs unless the seller can prove it. NIB (New, In Box) is a good thing, but even here, the price is usually less than in the store, though not by much.
I have bought things off Craig's List.
Here is how I compare different on-line stores:
1) Make a list of what I need, including sizes, for example quarter pound of fish food, not 2 pounds. Have some notes about priority and urgency.
2) Make 2 (or more) columns after that list, and make notes about each store. Price per pound for food (what is the difference between the 4 oz package and the 5 ounce? Both are close enough to the 'quarter pound' that is my target), price per dose or per gallon treated for water treatment products and any other way that you have to compare apples to apples.
If one store has a pint of dechlor that treats 900 gallons for 15.00 and another has a pint of dechlor that treats 8000 gallons for 15.00, then I would want to buy the second one. This sort of thing would be a deal killer: There is no way I would pay 10 times what I need to for dechlor. I would only shop where I could buy the cheaper product.
But if dechlor is low on my list (I may still have enough to last) and some other thing is more important I might end up shopping at the other store.
3) Actually start putting things in the cart.
4) Go to the shipping section. I find several sites have a button for 'estimate shipping', so I will use that.
5) DO NOT give them credit info until you know the bottom line. If that will not work, do not shop there.
Heavy equipment is so costly to ship that I will look around locally, a lot, before committing to on line shopping for something so expensive. At some point I say, 'It will cost me more in gas to take my truck to... (Wherever) so I might as well order it on line.'
Then you need to worry about who will be home to receive the package, if things are perishable, and similar concerns.
I have not found on-line coupons to be very helpful.