On eBay the buyer is making an offer against the total Buy it Now price including the BPF.  On other sites the buyer is making an offer against the price excluding the BPF (the amount the seller wants).  It really makes no difference. 

On the other sites if you list an item for £75 + BPF then any offer the buyer makes will obviously be less than £75 before the BPF is added, so no difference to an offer of £75 including BPF on eBay.  A buyer will never offer more than the total amount they are prepared to pay.