I purchased a lovely Burberry denim jacket (eBay find) that I had tried on in-store. The shoulders and hip fit well, but the upper torso area was a bit loose. The sales associate told me that it could be taken in.

As it turns out, the jacket can't be modified because the in-house tailors don't have the machines for heavy duty denim alterations. The tailor also warned me that the small alteration I needed would involve extensive work opening both the side and hip seams, cost a lot, and not necessarily produce a result I'd be happy with. She advised me to leave the jacket as it, since it fit well in the back.

TL; DR: Not all alterations are possible. Don't buy a product envisioning what it might be, buy it for what it is?

ETA: It's still a nice jacket and the only one I found that could comfortably accommodate by arms.