This article is missing crucial details, notably official confirmation on what exactly the substance in question is.
Even if it is Uranium, its value (and potency as a nuclear material) depends entirely on how highly enriched it is, or the ratio of U-235 (radioactive) to U-238 (harmless).
If it is highly enriched uranium, that would be valuable, and quite surprising. If the yellow color mentioned in the article is accurate, then even if it is uranium it is most likely unenriched yellowcake, hardly worth more than dirt.
Until someone tests this and finds out otherwise, the stuff they were selling is almost certainly worthless, and the only harm they are doing is running a scam.
Yellowcake is still radioactive correct? It could be used for a dirty bomb instead of nuclear weapons.
To quote from the wikipedia article on yellowcake:
The uranium in yellowcake is almost exclusively (>99%) U-238, with very low radioactivity. U-238 has an extremely long half-life, over 4 billion years, meaning that it emits radiation at a slow rate. This stage of processing is before the more radioactive U-235 is concentrated, so by definition, this stage of uranium has the same radioactivity as it did in nature when it was underground, as the proportions of isotopes are at their native relative concentration.
In short, no. It would not be effective or practical to use it as a "dirty bomb" to spread radiation. The levels of radiation it releases are simply not high enough.