Second marriages can be a tricky situation for a variety of reasons. There may be problems with step kids, ex spouses or even some religious stumbling blocks for the marriage taking place. However, when we talk about the etiquette for buying gifts for those going through a second marriage, it is a fairly straightforward process.
It is the norm for a couple going through a second marriage not to ask for, nor to expect, wedding attendees to bring gifts. Traditionally people received gifts at their first wedding to help them set up a home for their new life together. It is considered that couples going through second marriages already have all the trappings of modern life and do not require the gifts usually received at a first wedding.
If you have been invited to a second wedding and have received an invite that makes no mention of gifts, then that is good. It shows that the bride and groom have taken note of etiquette and have not requested gifts on their invitation. However, it is permissible and not uncommon to give gifts at a second wedding. If you are related to the future couple you may wish to give a gift anyway, if you are not married then you may politely enquire if the bride and groom have a gift registry, so as to make sure if you do by a gift for a second marriage, you get them a gift that they want.
Wedding Party Gift information and resources at http://www.1weddinggifts.co.uk.