I won't mention things other ppl have already :
Also, I don't know if it's such a good idea to have your main navbar set up in the middle like that, it kinda gets lost in the rest of the content. You want it to stand out since it's ppl will reference to. Either put it on the top, bottom, left or right it would make it look less cluttered and more professional.
Try rearranging your content on your home page. It's pretty busy page. Not that busy is necessarily a bad thing, but my eyes didn't know where to focus first. That's not a good thing. Make your most important point the most obvious to the eye then work your way down the list.
Try making your site scalable/resizing your graphics, I am getting horizontal scrollbars on certain pages
You've got your text centered on the overture page. Try removing that. It doesn't look right and that could also be making your graphic shift over causing horizontal scrollbars. If you align everything to the left you'll have a cleaner more professional looking page.
On your buyer's sign up page, the layout changes completely. Try to keep things consistent. If you're taking the consumers to a different server all together let them know. Same problem with the Search page.
I'd decrease the font size a bit. For your numbering system on the "how it works" page, they aren't very legible. Try using numbers for a bulleted list instead. <ul type="1"> etc.
I'd say ditch that grey strip you're using as a background. Either fill your entire background with that colour or don't use it. It looks silly as just a strip in the middle of the page.
DOn't forget to alt your image tags
How come you don't have any meta data?
I wouldn't use the inline font style tag. Stick to declaring your fonts and other styles in your CSS in the head of your document. Right now you've got both being declared. One inline and one in the head. They cancel each other out. Stick to one. If you want to have more than one font show up then set up classes.
Hope this helps

keep it up!
"bless us and love us so we can dance and truly be free"
- Anonymous