I am not saying that this post is only for experienced developers, a beginner can also check it only if they have API concept clear.
If not, you can check my API Guide: 

What is API? Why it is important?






I listed out the useful API services for E-commerce from programmableweb.com which is an API directory website.

1: Amazon Product Advertising API

Through this API developers can retrieve product information. The API exposes Amazon’s product data and e-commerce functionality. 

This allows developers, web site publishers and others to leverage the Amazon Product Discovery features that Amazon uses to power its own business, and potentially make money as an Amazon affiliate. 


API Portal Link:
https://docs.aws.amazon.com/AWSECommerceService/latest/DG/Welcome.html

2: Etsy API

API Portal Link: 
https://www.etsy.com/developers/

3: Shopzilla API

API Portal Link: 
http://publisher.shopzilla.com/

4: Magento SOAP API

Magento is a feature-rich, professional open-source eCommerce solution that offers merchants complete flexibility and control over the look, content, and functionality of its online store. 

The Magento SOAP API gives users access to functionality such as export/import of customer/customer addresses


API Portal Link: 
https://devdocs.magento.com/guides/m1x/api/soap/introduction.html

5: Best Buy Products API

Best Buy’s Products API provides full product information for over 1M current and historical products, including pricing, availability, specifications, descriptions, and images.


API Portal Link: 
https://developer.bestbuy.com

6: Bol.com Open API

Bol.com is the biggest online retailer from the Netherlands. Bol.com sells a variety of products, including DVDs, games, music, electronics, computers, and books. 

The Bol.com Open API allows developers to access and integrate the functionality and data of the Bol.com catalog with other websites, applications, affiliated programs, and e-commerce plug-ins.


API Portal Link:  
https://developers.bol.com/documentatie/open-api/ 

7: WooCommerce API

API Portal Link:  
http://docs.woocommercev2.apiary.io/

8: Walmart Open API

API Portal Link: 
 https://developer.walmartlabs.com/

9: BigCommerce API

API Portal Link:   https://developer.bigcommerce.com/api

10: Florist One API

The Florist One API lets you sell florist-delivered flowers on any website or application. 

Display flowers to your visitors in a fully customizable white-lablel storefront, have them purchase flowers without ever leaving your website – fresh flowers are delivered and you earn a 22% commission on sales.


API Portal Link: 
http://www.floristone.com/api/

11: Envato API

The Envato API allows developers to access and search Envato’s market information, retrieve user details, get Envato market stats, and check on the Envato forums from their own applications. 


Envato is a website that operates digital goods marketplaces and tutorial networks. Interactive documentation is provided for all of the API functions currently supported.

API Portal Link:   
https://build.envato.com/api

12: ClickBank Products API

The ClickBank Products API allows to create, read, update, and delete API operations. Methods allow to get a product, save a product, delete a product, and list all products in XML format. 

ClickBank is a digital lifestyle marketplace and an affiliate marketing network that delivers products to customers in 190 countries.

API Portal Link:  https://support.clickbank.com/entries/22821303-clickbank-api

13: Rakuten API

Through this API service, you can get data about products from Rakuten official online store hub and can able to customize according to your need.

API Portal Link:   
http://webservice.rakuten.co.jp/

14: ChannelApe API

The ChannelApe API returns JSON eCommerce data including inventory, orders, business intelligence, rate shopping, electronic data exchange, and channel management. 


Developers require an API Key to authenticate. ChannelApe provides Direct to Consumer (D2C) inventory and operations management software.


API Portal Link:   
https://docs.channelape.io/

15: Infeed API

The Intelligence Node Infeed API provides the latest retail data powered by machine learning, and artificial intelligence. This API offers endpoints for timestamps, orders, product details, discounts, shipping, inventories, and more. 

Most product identifiers such as Universal Product Codes (UPCs), Global Trade Item Numbers (GTINs), and barcodes are accepted.


API Portal Link:
https://www.intelligencenode.com/solutions/apis-data/

16: Aliexpress API by Aliseeks

It provides API services to Search products in real-time by image, item ratings, orders, category, and text with Real-time product information including the seller, item ratings, shipping, and product variations.

API Portal Link:
https://docs.aliseeks.com/