If it was permissible to buy the particular shares (depends on conditions like whether the business is halal and whether it has physical assets etc.) then it is permissible to sell them at any time.
Among the conditions for a valid sale (بيع) there is none that requires a minimum holding period, although you must ensure that you have ownership and possession:...
All forms of interest fall under Riba regardless of how they are calculated. If you gain interest from something it’s Riba and impermissible in Islam.
These articles might be helpful and discuss the various positions of the Ulema (scholars) and the Sharia regarding Riba:
The website clearly mentions the following solutions for Islamic finance.
No interest on contracts lasting longer than 24 hours – zero rollover interest consistently.
eToro doesn’t take any additional rollover commissions for contracts lasting longer than 24 hours.
We do not charge any commissions or fees for account management. Our revenues come ...