Hi folks! Welcome to our Forum!
Sorry about being tardy in answering but things have been weird trying to catch up a couple months backlog of paperwork. Anyway to answer your question, I need ask you a couple questions, What is the rifle he wants from you? How many rounds has he put through it? If it's in really decent shape as in the way you described it, personally depending on the number of rounds fired it's worth more than $500.00 USD, say somewhere between 600 and 700 USD, You have to take into account that it's a Pre-B from 1984, and he has everything that came with it. Well except for the extra mag. so deduct 50 to 75.00 for that. Your question about finding spare Mags is very good, and yes they are getting to be harder to find than a complete 75 Pre-B set. But you can still find them, even brand new factory issued one for about what Huddy mentioned. I have a CZ 75 Pre-B that I have had since the very early 80's and it's been my carry weapon for over 40 years. Just last year gave it to my daughter. NO idea how many rounds I have put through it but I don't think it was "thousands" but even so it saw a lot of range time, and I haven't had to replace a single thing. I have something like 13 or 14 different models of CZ's and backups for most of them, Cause I hate safe queens. If I buy one for the collection I really try very hard to find a second one to use as a carry weapon. Sometimes if they were used then I don't really bother looking for a replacement I just carry it.
Back to the topic at hand. I think with everything his 75 is worth more than 500.00 against a trade for your rifle. So if he is thinking your rifle is worth more than his CZ and he's going to get a bargin it may well be the other way around. Depends on the rifle and how much you like(d) it. Anything bought and paid for is worth whatever the payer is willing to pay. I have seen some CZ's go for stupid prices, but I have to figure the person that bought it really wanted it and it was worth the asking price. Who knows? So just get an idea how many rounds it's fired over the years, add in how many rounds you are going to be firing do a quick blue book check of your rifles worth, add in buying at least 3 or 4 more mags, unless you REALLY like loading mags. and decide from there.
Here's what I am talking about, at a gun show here in Florida I have seen NIB CZ 75 Pre-B's offered at $1000.00 USD and another NIB offered at $500.00 USD. (different times) Both sold on the same day the gun shows opened. So did the person that paid 500.00 for a NIB make out like a Bandit? Personally I would say oh ya. Did the guy that bought the NIB for a grand get ripped, I would say not really or at all.
No idea if this is in time to be of any help to you, or it confused you big time. I hope it helps.