Mate, if it's plastic glue you're pretty well rooted. Plastic glue works by basically melting the mating surfaces and bonding them into one part as it were. The only way you're going to be able to use those minis is cut them apart, file off the excess glue and then use the pieces. I've had to do this for a lot of minis I've bought 2nd hand over the net, it is a drag but as you point out, it'll save you a few quid.
Use a jeweller's saw with the finest blade you can get your hands on, this will reduce the amount of waste when cutting the minis, and remember Green Stuff is your friend when it comes to reassembling and/or covering up the previous owner's indiscretions.
Good luck with it.
If on the other hand they've been glued with super glue, hit the joins with some Zap-a-Gap Cyanoacrylate debonder, this stuff is the bomb.
cheers
duckman