Is it legal (in the US) to modify and resell a patented item? For example, if I buy a patented guitar pickup, substantially modify it, then install it in a guitar and sell it, is that legal?
You are asking about a doctrine called "patent exhaustion" or the "exhaustion doctrine". Basically, once you pay the license fee by buying a properly licensed patented article, the patent to the article is exhausted and no longer stands in the way. So you should be able to modify it. The more interesting question is repairs to patented articles -- for example, if a portion of the guitar pickup broke off and you build a new part, have you infringed the patent by fixing it with the new part.
I'd google "exhaustion doctrine" and "patent exhaustion". If that doesn't answer the question for you, you should probably ask a patent lawyer. Of course, as a practical matter, unless you're going to mass market the modified part, I don't know of any patentee who would sue an individual for infringing a patent with regard to a single guitar pickup installed on just one guitar.
If you do not modify it, and are a reseller of a legit made and sold item- no problem unless the manufacturer had you enter into a contract that said you can't - even then you probably can unless it is software. What are the modifications? Do they turn it into a new patented item? Is the method of modification patented? These potential problem patents may or may not not be owned by original manufacturer.