According to the Sellf's Terms & Conditions available here, you have 14 (fourteen) working days (from the date you receive the confirmation of your subscription via email) to withdraw from the contract with Sellf.
If you bought the Subscription with credit card:
to receive your refund, you must send an explicit statement by means of registered letter with advice of receipt to Sellf S.r.l., with registered office in Roncade, Tenuta Ca’ Tron, Via Sile, 41 – 31056, or to email address firstname.lastname@example.org or to fax +39 0422 789666, within this time period (after which such right is forfeited).
Should you fail to comply with the terms and criteria envisaged for the exercise of the right to withdraw, you will not be entitled to be reimbursed the amounts already paid to the Company.
If you bought the Subscription membership via GooglePlay:
please send us the transaction number of the payment you wish to receive a refund for. You find this number in the email you received after your purchase or in Google Wallet. Once we receive this information we’ll be able to issue the refund.
If you bought the Subscription membership via iTunes:
it can be refunded and approved only by Apple. To receive a refund from Apple you need to contact iTunes Support within 14 days from the purchase date. To contact them, please follow this link: iTunes Support. You should receive a confirmation email from iTunes Support when they receive your request. You'll receive a followup response with Apple's decision on a refund within a short period of time.
Here you can find a useful step by step tutorial on how to request a refund for an iTunes or App Store purchase.