I'm new to doing 3D art, and while browsing the marketplace I see a lot of content labeled as "not tested with DAZ Studio." Since DAZ is what I've been learning with, I'm wondering how big a risk I'm running buying any of this stuff?

Thanks for helping the n00b out...

Generally this caveat means the vendor won't support any issues you may experience while using the product in DS. You may find that materials need tweaking to look there best and in some cases complex materials won't work at all.

Also Poser dyamic clothing, Python script wont work in Daz Studio.

That said a lot of content will work fine in Daz Studio id even say the majority. Check the product comments and usually someone will drop a comment in about how well it works in DS or if ey experienced any problems. Also you can contact the seller directly and let them know you would purchase a DS version Or if ey know any reason it wouldnt work.

The Poser things that will not work are:

a) Poser dynamic clothing (as noted above) - not at all

b) Clothing with Poser shaders - the colours will look funny in Daz Studio but you can compensate using the Surfaces tab or if you have bought some Daz Shaders you can use those. This is the most common incompatibility but its inconvenient rather than a disaster. So if you see something where you really like the geometry its probably worth buying but be prepared to compensate for the textures.

c) Things needing Python scripts - won't work at all.

d) Older Poser characters like Miki2 will work in Daz Studio. New Poser characters e.g Poser 9 on will not but, especially if you have just started out, no need to explore Poser characters. They are not that good and its just novelty.

To be honest that common phrase 'not tested in Daz Studio' irritates the hell out of me because Daz Studio tries hard to allow Poser users to use their items but its not reciprocated.

If you are new to 3D and/or DAZ Studio, I would not purchase products that say "not tested in DAZ Studio".

As others have commented, many of these products will work in DAZ Studio or will work but the materials need adjusting for good results in DAZ Studio. Figuring out if a product will work in DAZ Studio is not something for beginners. Adjusting materials to work in DAZ Studio is not something for beginners. 

I would concentrate on learning 3D and DAZ Studio using content that designed to supports DAZ Studio. After you are more familar with material settings (the surface panel in DAZ Studio) you can look at some products that are not tested in DAZ Studio. By the way, you need to learn how to adjust materials to get the best renders in DAZ Studio, but this is not the first thing you need to learn.

Anytime you take a material into another program, you will probably have to adjust something. I export all my props and figures into Vue. I normally have to change highlight materials And bump maps. So using Poser items in Daz can present the same issues.