As Mart and Mark have said, I am a bit dubious about it being ex-RNLI. As Mart says, if it is ex-RNLI it may have been used as some form of safety cover, but not as a lifeboat - looks like a standard Avon S650.
Attached is a page from the brochure for the S650 (dated 1975).
Avons generally last very well, however the one on Ebay looks to be a bit of a mess - I wouldn't take it on unless you fancy a project. Some of the patches look pretty tatty and will probably need to be redone and the floorboards do not look original (not necessarily a problem provided they are a good match for the originals).