This is what my company does and it works well enough, though your site would need a reasonable amount of traffic or industry clout before an ad-buyer would take your email/call seriously.
Any insight on what a reasonable amount of traffic amounts to based on your experience?
I don’t remember specifically but I feel like we probably moved off Adsense when we were around 50k uniques/mo, but for a fairly specific niche.
One idea is to investigate who advertises on similar sites and examine their traffic/rates to see where you match up.