Though this may be the largest, richest and fastest growing market on the planet, common sense is still the most valuable local commodity. If it sounds too good to be true, it probably is. And if someone offers to sell you St. Petersburg pretty much had to pack everything with them as there were few things not in the tuber family available in the stores. Things have changed so rapidly that now almost everything you may need is available, though generally the more specific the item, the more searching it will take to find it.
Health insurance is never a bad idea for any traveller.
Speaking of which, it is good to bring some sterile needles and syringes with you in case of a medical emergency, as well as any prescription medicines you may need. During the rainy and snowy months the city can get quite gushy so some galoshes or good boots will keep the feets happy. Petersburg is that the names of many streets and locations are being changed. Appendix is a list of all the officially changed names.