So, the contractor made an offer (to you?) to do the work. Who accepted the bid? Did you pay the contractor direct or did you pay the seller? The problem here is working out who the contract to do the work was made between. If you did not pay the contractor direct, then probably the contract was between the contractor and the seller. In that case, you have no recourse against the contractor.
If you paid the seller, you probably have a contract with the seller. But what were the terms of that contract? If it was "a certain sum of money pays for an agreed amount of work", you have no recourse as long as the contractor did the work contracted for. If the terms of the contract were "a certain sum of money remedies all the rot", you have a claim against the seller, and either the seller and/or the seller and contractor between them should have taken the loss of doing the extra work. BUT 1) you have to prove this, which is likely to be difficult unless you have it all in writing, and 2) honestly, if you can live with the cost of putting it right, or live with not putting it right until you have the money, you have better options than trying to get recourse through the law. You cannot overestimate how expensive litigation is, firstly in money, but secondly and more importantly in time, energy, stress and diversion from the better things in life.
WARNING: you don't say what legal jurisdiction you are in, so the law may well be different where you are. I've just given you the basic common law position.
Short of taking action, you can either write to the seller yourself, or get your estate agent to talk to the seller or the seller's estate agent, to try to get some sort of remedy.
Lesson for next time is: either get the cost of the renovations taken off the price and arrange them yourself, or get a contractor you trust to do the work and inspect it before signing off on the purchase.
