should i have sold my collections?

got the person wanting to buy my collection for around $1100ish.
i turned it down. i think i'm on about 7th family.
bad move?

If you are going for the collection trophies, then you'll probably want to wait until you have a complete collection before selling. However, if you are in desperate need, then the collection can offer you some quick cash. Of course, the longer you wait to sell your collection, the more money you can make on them.

so, it's not a one time offer?

It's a random game event. It could happen again, but there's no guarantee that it will.