A little more info would help us give suggestions. Such as your country, if you just need the Cart & Checkout to use with an existing website, or if you want a full blown ecommerce system that turns your catalog items into product pages.
A couple of suggestions:
Pick a Cart provider based in your own country. That will make it easier to understand the documentation and to reach their tech support.
If you already have a website that you just want to add BUY and VIEW CART buttons to (or want to build that type of website) then your choices are more limited. The only US based cart providers I know of that will allow this separation of Cart and Site are UltraCart and AmeriCommerce. I am using UltraCart for one of my stores, and I know it has lots of settings for wholesale and quantity based pricing (though I do not use those settings since I do not wholesale)
*I prefer this type of solution because you have much more flexibility and it doesn't look like every other ecommerce store out there. You can see my stores at http://www.bestdryingrack.com (uses UltraCart) and http://www.easydigging.com (uses Yahoo Merchant Solutions)
If you want a full blown ecommerce provider system, my only suggestion is to experiment (this will cost you a little money but save you much frustration later)...
1) Research enough to make a short list of possibilities
2) Create a mini-catalog with two main categories (fruits and vegetables) then two sub-categories for each category (apples and oranges, beans and peas), then two items for each sub-category (Red Delicious and Rome, etc)
3) Sign up for your first choice (so you can get tech support) and try building a very simple store using your mini-catalog.
4) If you are having major problems loading your mini-catalog and setting the pricing and quantities to match your wholesale desires, call their tech support and see if they can help.
5) If not, stop. Save your work and keep your subscription, but go to your second choice and try the same thing there.
6) Once you get the catalog working like you want. Move to the page building stage. Try building a basic Home page, SubCategory page, and Item page.
7) If building pages in the temporarily winning ecommerce system isn't working for you, then you may have do like you did in Steps 4 and 5. Either try the next possibility on your list or go back and re-try one of the earlier abandoned systems.
The whole process is often very aggravating. be prepared to pull out some hair :rolleyes: