Restoring damaged paper wallets? Is that what you mean?
And what kind of damaged you can restore?
It depends. Could have missing end, missing beginning, unreadable letters. Each case is different.
What's your website where you operate? Why I ask? Most legit bitcoin recovery services have a website.
Some
non-legit have a website too ;-) If fact of having or not a website decides...
What are the conditions for selling paper accounts and wallets? Is there a way to disclose parts of a private key without losing money?
It is a risk for both sides. I never know if you sell the wallet for the address mentioned; you may transfer coins just after you sell wallet to me and then sue me. We may sign the contract where you declare that you have no access and you did not distribute the wallet to third parties.
If you do not want to sell but just request opening and pay fee (depending on difficulty), I may give you VAT invoice.