As Burgie has already said, you have a legally binding contract with the seller at the level of the winning bid
.
That's the good news though
In practice ebay are likely to be very little help in enforcing this - yes, you can raise a dispute and they will take their own sanctions against the seller, but there is little they can practically do to force through the deal. If you really want the caravan and the seller doens't want to sell then you would have to start legal proceedings.
It may be that, as pugboy and burgie have said, there's something suspicious about the whole deal - but it's more likely that the seller just hasn't got as much as they wanted. I'm not a lawyer, but I strongly suspect that the advice you would get at this stage is to send a polite and friendly reply saying basically "thanks - but no thanks" - that you still want the caravan and are happy to honour your winning bid. Don't mention legal action etc at this stage - just leave it at that and see what the seller says.
If she comes back and says no deal - at that stage you should then politely remind her that you have a legally binding deal. Whatever you do though keep it polite and friendly in tone - if you do go down the legal route later then getting ars*y now may count against you.
Good luck and let us know how you get on