If an end user contacts you and you reply right away, I think it’s a bit of a mistake. Appearing too eager to sell is not exactly a good idea. I understand that it’s tempting to reply right away but I’d recommend resisting the temptation.
On the other hand though, I exchange emails with domainers on a regular basis and whenever I do business with a fellow domainer, the previously mentioned principle doesn’t apply. I mean we both know about the “don’t reply too soon” end user sales concept, so the “competitive advantage” or whatever you want to call it is just not there.
That’s why I simply reply when it’s convenient because in my opinion, the “anatomy” of the sale is completely different and delaying replies does more harm than good. Deals between two experienced domainers can be finalized after nothing more than a virtual handshake so again, the anatomy of an end user sale and the anatomy of a domainer to domainer sale are completely different.
All in all, I’d say that:
1) if we’re talking about an end user than yes, replying too soon is a mistake
2) when it comes to domainer to domainer sales, it’s not that much of a big deal