I've found the same thing. I list my sales at DNF, NP, and DS. It's more upkeep, but the other forums provide more sales, and the people there are more inclined to buy on impulse whether than looking over the implications of the domain they are purchasing.
It is true that DNF generally has more professionals, but I wouldn't say that everyone here is professional. Lately I've been seeing a lot of inappropriate posts and childish bickering. But all in all, yes, people here are more discriminating. For a seller, this means you may have better success elsewhere, but if you have really spectacular domains, DNF is the best place
I like your modelingtalk.com set. I may have made an offer on these. However, the others have some core problems:
GadgetsTalk.com - GadgetTalk.com would be much better. Plural nouns don't come across well as adjectives, so "Plural" + "talk" is not really proper English, and has little value as a domain.
SolidHosts.net - Would be great as a .com. As a .net, this is less than mediocre.
HostReviewWatch.com - Who needs a site to "watch" the "host review" sites? Isn't that fairly redundant? HostWatch.com or HostReview.com would be great, but this domain is simply worthless.
HostReviewsWatch.com - Same here, but the plural makes it even worse.
SpamUp.com - Fair brand, but people on this board don't value brands very highly. They tend to go for keywords, links, and typos (in other words -- traffic).
So, you see, it's nothing personal. Just keep at it until you find what people are looking for. Good luck!