- The BSD license requires one to preserve the copyright notices and the BSD license. If that person stripped these from the software, he would be in violation of the BSD license.

- If the seller did not add anything that is copyrightable, I guess it's questionable if his added terms hold up. In this case it is exactly the same as the original BSD-licensed product, and covered by the BSD license.

What usually happens is that some proprietary software vendor takes a chunk of BSD-licensed software (say, a TCP/IP stack) and integrates it into their proprietary software (say, a kernel). In that case, you'd theoretically be allowed to pry out the BSD-licensed parts and redistribute it. In practice, this will be impossible, since the original source files are probably changed to such an extend that it's a mixture of BSD-licensed and proprietary software.

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Of course, this is all the effect of choosing a weak copyleft license. You are practically saying 'you can do whatever you want with this code, as long as you list my name and this license in the source code and/or binary versions, and don't sue my if it doesn't work correctly'. If you want stronger protection, you should use a strong copyleft or a proprietary license.