My experience is that most people will say thank you for answering their query whether it is by PM or on the open Forum. However, there are a significant minority of people who regard assistance on internet forums as being a free public service and will not acknowledge that it has taken time and trouble to answer their questions. They are probably the same people who download music tracks without payment on the basis that 'everything on the internet must be free'. There's not really much that can be practically done about this.
To some extent, most of us are guilty of plundering the net without giving anything in return when we seek information by browsing other forums, not just model boat related ones, but this is a passive activity and doesn't actually require anyone else to put themselves out specifically. We contribute by sharing our own expertise on this forum which is free to read by anyone.
There are ohter people who use this forum solely to make a private sale of a model boat, either theirs, or maybe their deceased Dad's. As long as it isn't a commercial post I tend to let these stand on the basis that there is a potential benefit to our members who might be interested in the item for sale.
And then there are people who register with every single MHS magazine forum to give them the maximum opportunity to win prizes in our competitions which they can then sell on Ebay! That is why the better prizes are offered to magazine subscribers only as we can be sure that these people are actually interested in model boats.
I suppose, as the saying has it, 'There's nowt so queer as folk' and a lot of them are after something for nothing.
Colin