Help with Forms and the Gallery

So I have the gallery setup like a shop where they can buy the images in it. I want to know if there is a way to set this form to go to a topic instead of private messages(which overflow at 50 which I can easily get in a few days). Someone said it's possible to change this form so it goes to a topic if I want to so if someone could help that'd be great...

Code:

<form id="privatemessage" action="/privmsg" method="post" name="post"><input name="username[]" type="text" style="display:none" value="Leandra"/>    <input name="username[]" type="text" style="display:none" value="SweetShelli"/>  <input name="username[]" type="text" style="display:none" value="Chloe"/><input maxlength="64" style="display:none" name="subject" type="text" class="usernameinput" value="Transaction: {PIC_TITLE}"/>  <textarea id="cartMessage" style="display:none" class="noThis" name="message" >[center]Congratulations, {PIC_TITLE} from {POSTER} has been sold. {PIC_DESC} Points have been deducted and the fashion is attached below. [hr]<a href="http://www.designerssociety.net{U_PIC}" download="{Poster}">[img]http://www.designerssociety.net{U_PIC}[/img]<br><span class="sendingBut" style="color:#fff;">Download Fashion</span></a>[/center]{PIC_BBCODE2}      </textarea><input type="submit" class="sendingBut" name="post" value="Buy Design" /></form>

Now I also have a form like this that is used for making offers for a certain amount of points. I have the description set as the points amount, and Id like that plus the buy button above to be hidden if the description is blank. So basically the item isn't for sale if the seller doesn't give a price, and members can make offers. Is there a way to do this???

Help with any of this would be really great, as soon as possible please
Also if needed here is my site, you'd need to join to access the gallery though, I can give any codes needed though too.
http://www.designerssociety.net

Help??

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