Is it possible to import a list of customers into BUSY? If yes, what is the process?How do I generate bills for different sales platforms like eBay, Amazon etc?

Yes it is possible to import a list of customers into BUSY through excel and xml etc. 
Follow these steps to import a customer list into BUSY through excel:
Step 1: Go to administration menu
Step 2: Then click on data export import option
Step 3: Then click on import masters from ms excel option
Step 4: Then click on accounts option from that window you can import your party/cutomers ledgers in BUSY after maintaining a data in excel
If you want to import masters from another company that you are using in BUSY
You can follow these steps: 
Step 1: Open your company from where you want to export the data 
Step 2: Then Go to adninistration menu
Step 2: Then click on data export import option
Step 3: Then click on Data export/import(XML) option
Step 4: Then click on export data option then click on masters and select the account and set the file path where you want to export file then proceed for the export
Step 5: Then open the company where you want to import that data 
Step 6: Then Go to adninistration menu
Step 7: Then click on data export import option
Step 8: Then click on Data export/import(XML) option
Step 9: then click on import data option then give the file path where you exported the file and proceed
You can create a bill for different sales platforms like-eBay,amazon etc
Follow these steps to generate E-commerce bills in BUSY:
Step 1: Go to administration menu then click on masters option 
Step 2: Then click on sale type option then click on add
Step 3: Then create a sale type mention tax rate according to your requirements 
Step 4: Then enable the sales through E-commerce portal option as yes and specify the E-commerce portal username 
Step 5: Then go to the transactions tab then click on sales invoice then click on add 
Step 6: while passing the entry select the sale type you have created for E-commerce and proceed to generate a invoice