$75 is fair bid for a complete gut depending on the size and condition. Growing up in Eastie, I know these homes very well. It may be more advantageous to sell outright than get in over your head with renovations. I've got so many investors paying top dollar even for complete gut homes.
I also applaud the agent. If she's trespassing on the property then you could report her. If all the owners say "no, they're not looking to sell", she won't be around the building anymore and just move to the next one.
It seems the owners don't need to sell. It could be rented and the owners are holding the property as an investment. Your best bet would be to move on. Best of luck,