It really depends on what your contract states and what you have in it for contingencies, if these request come as part of an appraisal condition or a building inspection, then yes they can walk, however if they are just trying to re-negoiate, then you are not required to give them a credit. I do agree with you, that things that are obvious like paint during an initial walk thru should have been negotiated on the original purchase and sale agreement. Hope that helps...good luck!!
Most Cities and towns will have an assesed tax value that is considerably higher the market value. If you had an appraisal for $40,000 and it was done by a certified appraiser I would go by that. Your best offer is typically your first one you get...the longer it stays on the market, the less they will get...good luck!