I have good news and bad news. The good news is that you've found a collection of very knowledgeable and fanatical Port drinkers!
The bad news is that you've found a collection of very knowledgeable and fanatical Port drinkers!
None of those Ports are particularly rare with the exception of the 1952 Niepoort, which is a type of Port called a Garrafeira. Those are pretty rare and can command a fairly high price. Unfortunately the rest are either common years or second tier producers/Ports. I'll give you approximate retail value in the US, but you should not expect to get that in a private transaction. Generally you'll get 60% to 70% of retail for a private sale in the US unless you have a relationship with the buyer or some sort of documentation regarding the provenance of the Port.
1. Offley 1980 "Boa Vista"
1980 is a generally declared year, but Offley isn't a top tier producer. It isn't something that you can normally find easily, but that's more because it was all consumed back when it was in its prime than because it's being hoarded for future drinking. You'd be very lucky to get $100 for this in the US, and the $60 range is more likely. Don't get me wrong - it's a fine Port - but it's just not all that great.
2. Cockburn's 1963 "Vintage Port"
One of the better Ports on your list, and from an excellent year. These will hold for quite a few more years but are also superb for drinking now. Probably worth around $150 in the US, maybe more for someone born that year.
3. Graham's 1976 "Malvedos Vintage Porto"
1976 was not a generally declared year, which is why Graham's produced a Malvedos that year instead of a fully-declared Graham's Vintage Port. That said, Malvedos Ports are generally very good or better so this will be a tasty treat for whoever eventually gets to drink it. I'd value this bottle at roughly $100 in the US.
4. Dow's 1977 "Silver Jubilee Vintage Porto"
Another of the better Ports on your list... when the bottle is sound. Unfortunately 1977 Dow has an unreliable reputation which has limited its value on the secondary market. Retail prices vary wildly in no small part because many wine retailers don't really know much about Port and so price it based on its name and the year alone. (Other top-tier 1977 Vintage Ports sell for around $150/bottle.) Those that do know something about Port will have marked it down to the $100 range due to the problems with this particular Port.
5. Niepoorto 1952 "Port" - decanted 1974
The gem of your collection. Garrafeira is not a widely-known style of Port, but it has a strong following. Niepoort is also not a large producer, so their volumes are low, but they are generally well respected. A 1952 Garrafeira can command $350 - $400 at retail in the US, but sales opportunities are rare due to most people's lack of knowledge about the style. It takes a true aficionado to search out these Ports and be willing to pay the premium required to acquire them. Due to their legitimate rarity you may find much higher prices listed on the internet, but I would caution against getting too excited when you see them. There's a reason they're still listed at that price... it's because they haven't sold. These are often "trophy bottles" in a store's collection that they list well above expected value so that they can have something old and seemingly very valuable to display.
6. Deleforce 1975 "Finest Vintage Port"
7. Delaforce 1977 "Vintage Port"
8. Delaforce 1978 "Quinta Da Corte Bottled 1980"
9. Delaforce 1978 "Quinta Da Corte Bottled 1980"
Delaforce is a second tier producer. Third tier might be closer to accurate. 1975 and 1977 were generally declared years, but 1978 was not. And 1975 is generally regarded as a low-end general declaration for a variety of reasons not germane to this discussion. As with the Offley above these may appear to be rare and/or hard to find, but it's more because they're now past their prime and have mostly been consumed than because they're rare and valuable. I would value these bottles at maybe $60 to $80 each in the US, though you'll likely find wildly differing estimates on the internet. And you'll probably have difficulty finding anyone actually interested in buying a Delaforce because they're generally just not that good.
Hope that helps!