I don't feel qualified to give an opinion on the strand but have a few thoughts.
If it was me, I would prefer to buy from someone I felt sure was selling me the real deal. You can always check on the various online sellers for their lowest price. I agree that second-hand can be cheaper but I would be worried about what I was getting, especially if it seemed like an amazing deal.
A long time ago a member here, David who makes incredible pearl creations, suggested this auction site for people in Europe (I noticed you linked to UK eBay):
https://www.catawiki.com/c/313-jewellery
I think he said they sometimes sell there. Catawiki have dedicated pearl auctions and I would be more likely to trust them than eBay. I believe there are some forums in the US where people sell their own pre-owned jewelry.
Another option is to go for barogue / circled Tahitians from inexpensive Etsy sellers like KongsPearl or Wen Pearls (not sure I got that right) as you will know that you are getting what you see in the photos and Tahitians have a way of being a pearl you can always use for _something_, even if you grow into better quality.
Third option, remembering how some members here had to try many Tahitian strands before finding one they loved, is to buy from a seller you can return to. That could save you a lot of money in the long run, compared to buying good quality Tahitians second-hand and not being able to return if you don't like them. One UK-based seller who is a member here is
https://www.pearlescence.co.uk.
- Karin