They are two different things - a reseal uses original components and is basically the original bubble 'resealed' back to the card - normally using some sort of glue or double sided tape
A repro is normally all reproduction items - card and bubble (and normally weapon too) - these are made of different card stock and are normally quite easy to spot - especially alongside original cards
The best way to learn about the differences is to buy a few beaters and cardbacks first so that you are familiar with them. There are also a great number of people on here that would be happy to advise if you can provide pictures