Welcome to Bladeforums!
Yep, $25 (gold membership) or more (plat is $75?) allows you to posts knives for sale in the For Sale section. It also allows you Private Messages (which is nice) and an avatar. There are NO standard costs like the 'bay for posting an ad or any commission due upon sale of a knife. Make sure you read the stickies (rules) in the For Sale thread.
Your For Sale thread can stay active as long as you have stuff to sell in it. Like BH said, only bump it ever 72 hrs to the top of the list, else you anger the mods. Be detailed in your description and pics always help.
If you enjoy the site, I think $2+ a month to support the site is worth it.