Any reputable domain registrar has a "push" function. The 60-day hold is only for transfers OUT to another registrar. A "push" means it is transferred to the buyer's account at the registrar where the domain was registered.
So as an example, if you have a domain registered at, say, NameCheap, but your 60-day transfer hold is not over yet, you can still sell the domain, and then push it to the buyer's Namecheap account. If your buyers doesn't have a namecheap account, then they have to create one before you can push it to them.
After you push the domain to them, if they want to transfer it out of Namecheap to another registrar, that is their business & responsibility but they will have to wait until the 60-day hold is over.
If they need help on how to transfer out, that really isn't your concern but you can help them if you are so inclined. I would only suggest this if you sold a domain for a high value to an end-user that isn't familiar with domain processes, especially if you have good reason to believe that buyer could possibly buy more domains from you in the future.
Otherwise, don't get involved in a transfer processes and just push domains to your buyers instead, even if the 60-day hold is over. Transfers are really the owner's/buyer's responsibility. Besides, their new registrar they are moving to will likely help them with the transfer since they are gaining new business, if they ask support to help. This is assuming they are transferring to a reputable registrar. But again, this is none of your concern, really.
3 registrars that I have used in the past 10 years who allow for pushing are...
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NameCheap
https://www.namecheap.com/support/k...-domain-from-one-namecheap-account-to-another
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NameSilo (coupon code NP1OFF for $1 off you first order)
https://www.namesilo.com/Support/Domain-Push
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Dynadot
https://www.dynadot.com/community/help/question/domain-push
https://www.dynadot.com/community/forums/f7-domain-name-help/transfer-domain-push-question-1678.html