Are there other properties on the street that have the same situation with the pipeline? Have any been sold? Have your agent get those comps and also comps for this house and you can then see what it does to the value of the house.
On the other hand, it could be your great "reason" to negotiate the price if many real estate agents believe so. Say, if they are the listing agents of this property, and you like it so much, and you catch this "sort of problem, but not really a problem", you can point that out, and the seller's agent will be on your side and urge the seller to lower the price for you.
If the "sight" of the yellow marker bothers you, plant some nice trees to hide them like I put some nice bushes to hide my a/c and cable box ...etc, and some friends hide their heating oil tank, wow, right next to their house, 10 ft? Technically they won't hurt you because otherwise, the house would not have been permit to build at the first place.
On the other hand, there are so many nice houses available http://cnj.craigslist.org/search/rfs?query=piscataway&mi you can take your time too. I would say, make an offer of this house "at your lowest price" that you feel compensated psychologically, and in the mean time, do not stop your housing hunting, continue going to open houses (do not go with your same realtor to see other houses as there may be interest conflict, your realtor making offer for this house may either want to wait for the result or want you not to get it altogether). If you can not find better, say, in terms of location, school, train, ...etc, or the seller accept the low offer that you feel great, you will be happy to get the house.
If you feel great with the below the market price to get the house, you know your future buyers will feel the same way. I have not seen any house can not be sold in Piscataway or any other town, but price is the key, so a few % lower would certainly help, now and later....
I used to buy a land that nobody wanted because of high tension tower, so I drove up to dozens of millionaire towns in NJ and found out many million dollar houses right under high tension towers. So, I told myself, if millionaires do not care about high tension towners, I would be fine. Years later, I found I was correct, the whole area now built million dollar homes, ironically, the one log right under the high tension town built the largest one over two millions.
I could never understood why then I finally realized one day, for middle class families, they will stay home for most of the year except weeks of vacation; however, this is NOT true for millionaires. e.g. my brother-in-law is General Dealer of many cruise lines to China, he only spend about 50 days a year in his $6 million dollar house in San Marino CA. When he came back with an open Christmas party with over hundreds of guests invited, live band, ...etc that was one of the good purpose to own house like that, but who can see high tension town at night? Besides, he is not around most of the time, even if high tension town may cause real problem which never confirmed, the real saving on real estate and tax ...etc is very real.
There is another type of property with "easement" like other experts mentioned. If you got something like that, it would be wonderful that you can charge money on it. My friend's land charged $100,000 to a company want to connect sewage thru his land.
Ah, I got a land that has a river passed thru that no body wanted because the thing called "conservation easement" that you can't do anything due to webland conservation. But, you know I wanted it, I wanted it badly. Why? you can deduct huge amont of tax. This is also true for historical building ...etc. However, most realtors, if not all, would not know this, not even many tax lawyers.
This house you described, I immediately would think of it can get a great bargain since many realtors already on your side. Hey! like you take some drivers to court, and their lawyers all on your side, so do not hesitate to sue for millions. Once your bargain down the price, you would go to tax court to appeal down the property tax, and they will all lose, if you bring in your realtor, and realtor said this property is overestimated because the above mention reason, hard to sell ...etc. I mean, this is the situation that you pay, say, 10% lower than your neighbor homes, and you got a proof.
As always, I do not suggest anyone to make decision base on my sharing of experience because I can only tell based on what you said. e.g. someone told me his condo is fantastic, the only problem is that there is noise of train once in a while. I said of course because you want to be able to walk to train in just a few minutes. So, you can always has a perfect home. If there is one, someone may have taken it already, or you may have to pay too much to get it...
If there are no recent EPA or pipeline problems, the impact will be slight. If there is a problem w/ a petroleum pipeline anywhere and it is highly publicized within a year two of when you might sell, the impact could be substantial.
Pipelines do have a storng safety record.
also, you can contact the nj epa dept for more info.