There will always be interest in G1s!
However I understand the hassle of listing so many ponies all at once!
As much as its a pain to take pictures and write descriptions of 70 ponies; if you dont do it at the start you are going to end up doing it as you go and that takes way more time and effort.
However if you dont want to sell them individually, I would think it would work better for you to at least break the 70 total down into smaller lots, 10 or 12 per lot perhaps. The smaller the lot the more per pony price you may be able to get - this is worth while as typically people want to pay less per pony for lots than individually.
You could group them by condition perhaps? All the "poor" ponies, all the "fair" ponies, etc or simply by type of pony - whichever is easiest for you.
I would suggest at least a one liner per pony per lot on any glaring issues - marks, hair cuts, etc. Whether you are selling as one large lot or several smaller ones.
And if you are selling them on the Arena simply set your price and stick with it or run as an auction but set a reserve that you are happy with.
Good luck!