Well I bought the Mav for £340 on ebay as spares or repairs and it steamed over winter a little just after I bought it. Gone worse now though. I have had many cars from ebay and as a rule of thumb if its cheap it probably has a hidden reason for sale. If I sold it on ebay, I would mention the heater, but I fear not many potential buyers would recognise the difficulty of sorting it.
If it goes, it would probably go traded-in and I wouldn't bat an eyelid about it.
At the £500 and below end of the ebay market most buyers would expect some hassle I think. And other than the heater, it runs and drives very well, so a purchaser could not grumble.