When you talk "Sell-Thru-Rates", you are mainly referring to large portfolio holders that play the bulk sit and wait game. They may or may not list a bunch on various marketplaces and then wait for someone to inquire about one (Leveraging the marketplaces marketing to get eyeballs visiting the listings).
Occasionally, they may do a little outbound, but not as often when it's a larger portfolio, since outbound can be very time intensive.
In short, if you are only focusing on large portfolio holders playing the sit and wait game, then yes, under 5% is normal.
That % goes up with the more time you put into it, proactively (E.g. outbound, social shares, paid advertising, free advertising, brokers, coops, partnerships, etc..)
Even then if you're not targeting the right potential buyers, not pricing competitively, fail at negotiating, pitching low-quality domains, etc... the extra time could simply be wasted effort, but still a learning experience.
Everyone does it differently.