You will pay closing costs on your AKV purchase. You are already going to be paying closing costs on your resale. No closing costs will be assessed on the AKV purchase, if it is an add-on. If you have an extra $350-$400 you don't have any better use for, this may not be an issue for you.
If you are already in the system with VWL, Disney will be certain you get the same use year so that you can have one master contract. If you purchase AKV while waiting to close, you will need to make sure that you get the same use year for AKV, then make sure the agent and the closing company are aware that you already have a master contract, so that the VWL purchase can be closed under the same contract number. Otherwise, there are complications possible, when using the points. (Points from both contracts could not cover a single night's reservation without a transfer, which is a PIA.) If you NEVER plan to mix your points - i.e. if you will use all applicable points at the 11-month mark to get the reservation advantages for each of the two resorts, this may also not be an issue for you. (I notice 4 posts - but assume you are aware that you can only use the points that attach to each particular resort at the 11-month mark, and could only "mix" the points for use at the 7-month mark.)