When planning the design of a Custom eCommerce website design, several key factors should be considered:

1. User Experience (UX): Prioritize intuitive navigation, clear product categorization, and easy checkout processes.

2. Mobile Responsiveness: Ensure the site is optimized for various devices and screen sizes to accommodate a broad user base.

3. Loading Speed: Optimize images and minimize unnecessary scripts to enhance site performance and reduce bounce rates.

4. SEO Integration: Implement SEO best practices for higher search engine rankings and visibility.

5. Security: Employ SSL certificates, secure payment gateways, and robust data encryption to protect customer information.

6. Product Presentation: High-quality images, detailed descriptions, and customer reviews contribute to informed purchasing decisions.

7. Payment Integration: Support various payment methods and ensure a seamless, secure transaction process.

8. Scalability: Design for growth, allowing the site to handle increased traffic and product listings over time.

9. Content Management System (CMS): Choose a CMS that aligns with the business's needs, enabling easy updates and scalability.

10. Analytics and Reporting: Integrate tools to track user behavior, sales, and conversions for data-driven decision-making.

11. Compliance and Regulations: Adhere to legal requirements, such as GDPR, for privacy and data protection.

12. Brand Consistency: Reflect the brand's identity through color schemes, fonts, and overall aesthetic.

13. Customer Support Integration: Include features like chatbots, FAQs, and contact forms to assist customers.

14. Social Media Integration: Enable easy sharing and connect with social platforms for wider exposure.

15. Feedback and Testing: Gather user feedback and conduct thorough testing to identify and rectify any usability issues.

Considering these factors ensures a well-rounded, functional, and user-friendly custom eCommerce site that can drive sales and foster customer satisfaction.