Ideally you will always use comps from the same subdivision, same village, etc. When you get into some areas that have more limited comps, you need to use the best comparables which may be from another village/zipcode. The comps that you are most likely seeing will be Riverwoods which shares Deerfield's zip 60015 & can be very similar to Lincolnshire.
Like Kathy said, you really need to get an agent to represent you on the buyside. Just because you went into an Open House does not mean that you have to use the Listing agent as a Dual Agent....to do a dual agency, there are numerous disclosures that both buyer & seller have to sign so everyone is very clear that the agent is representing both the buyer & seller.