This is rubbish. I am (was) a serial domain buyer. Now, I guess its somewhat true that buying a domain to go with your idea can really inspire you, I have a lot of domains I let expire or still hold on to because I haven't yet or never did anything with them. I don't think its a good idea.
In all the time I've had a bunch of domains there's only been interest in one of them `spare.io`, for which I've had multiple unsolicated offers.
(That despite the fact I have two forename-based domains.)