We purchased our TT 3 years ago in California. We got the title from the seller and she signed it. It looks like we were supposed to have turned it into the DMV within 10 days, but we were crazy busy ...
I've sold vehicles/toys without having transferred the title to my name. Signed title is a signed title in my experience and I've also been able to title vehicles I bought that were not titled in the sellers name.
Each state may vary and it may be an uncomfortable conversation for you with the buyer, so you can either title it in your name first and they can winder why you just titled it last week, or you can explain you never titled it. Nothing to worry about IMO.