Whilst I understand your well meaning , behind this warning , how would it help me and the majority of sellers who sell Buy it Now items.  There would just be no time to read any ' warning' feedback,   and the buyer would be free to come and buy from me.

The main problem a seller can come across,  is a non payer. 

Feedback here as a warning just will not help,   as above for sellers selling Buy it Now items,  but if the tools provided by eBay were used correctly , that is,  opening those unpaid item cases, unpaid item strikes are applied and serial non payers cannot buy from the majority of sellers on eBay, and can eventually even be banned from eBay's site.

Cases are effective,  feedback isn't.

Btw,  this is a ten year old thread,  best to open a new one after that length of time.