The TSN is composed of 4 groups of numbers (www-xxxx-yyyy-zzzz). The first two groups (www-xxxx) are generic across that model of TiVos - the first group identifies the TiVo, the second group identifies revision. The last two groups are close to a serial number.
I believe it's safe to provide the first two groups of numbers - it'll help identify the TiVo to the buyer without revealing the whole thing. The danger of providing the whole TSN in your case is because it's never been subbed is the buyer could theoretically sub it and prevent you from selling it because TiVo will not allow anyone else to register it until it's been deregistered. They'll be paying for service, but given how crazy some people are...
Edit: I mean, it'll identify the TiVo model, without identifying the individual TiVo.
I believe it's safe to provide the first two groups of numbers - it'll help identify the TiVo to the buyer without revealing the whole thing. The danger of providing the whole TSN in your case is because it's never been subbed is the buyer could theoretically sub it and prevent you from selling it because TiVo will not allow anyone else to register it until it's been deregistered. They'll be paying for service, but given how crazy some people are...
Edit: I mean, it'll identify the TiVo model, without identifying the individual TiVo.