How can the gap between sales and after sales service be reduced to retain customers? Any suggestions?

 By build strong call center and customer care service

This is a very open ended and weak question. To be able to provide the correct answer we will need to know what is your business product or service? What's your company size? Do you do retail or wholesale or B2B selling? Who is your customer?

You can't expect people to give you advice on such an open ended question. Sorry.

The gap can be closed by getting both teams on the same room regularly and reminding them of the bigger picture until they learn to identify themselves with the product of service.

The moment the Sales believes the truth - Aftersales brings Sales

Internally by designing a mechanism which allows meetings between sales and support team.

The same mechanism should capture Voice of Customer and keep it as an agenda of meetings.

Marketing should involve to push Customer engagement and give its input for these meetings.

Most of the organisations / customers suffer because of this and the blame game starts.

For handover:

I always recommend a long transition depending on the product to take place and the sales team cannot just leave after the sale until the customer ans service sales team is satisfied on papers and takes responsibility for all future complaints in agreement with the customer.

For getting repeated sale: The service sales person shall be rewarded for any repeated sale in his territory like sales incentives shall be shared. also there has to be a firm line saying what is new sale ans what is after sale. 

Indeed, in many businesses sales has only attention for new customers, and after-sales does the troubleshooting. Existing customers are neglected or even disadvantaged. (Because of this attitude Electrabel lost almost50% of its customers)

Studies show that losing a customer costs much more as attracting a new customer. So for sales it should sales it should be a key mission to target the existing customers with (for example) new offers and fidelity benefits.

By implementing CRM tools (either manually or through a CRM system) to plan services to customers and track customers' complints.