It looks like you registered as a freelancer on Upwork. Please, check out this help article for information regarding reporting your Upwork Income to your local tax authorities. Note that Upwork is able to provide documentation only to some freelancers, however, such reports as Certificate of Earnings and Earnings by Client are available to all freelancers under Reports. Please, talk to your local tax advisers about using that information to report your income.
According to the TOS, you are not buying anything from Upwork. You are buying directly from the freelancer, through Upwork's financial/escrow services. You get an invoice with the freelancer's name on it as 'the seller'.
The same issue occurs on the freelancer's side; officially they are not being paid by Upwork - they need to issue invoices to the client directly, but on the bank statement, the money appears to come from Upwork.