I actually can't tell. I think you should do a little research about that. Check online (Amazon) to see what the costs of the products you're looking for are, then compare that on Nigerian popular ecom websites like Jumia and Konga. That will give you an idea of what the costs would be.
Afterwards go back to Jiji and check what the sellers are offering. You can even get it lower since there are a lot of sellers there.
About getting rekted... Yeah, you should be careful when dealing with sellers on Jiji. You can always check their customers reviews, get their contact info, and visit their offices/shops to make sure they are legit. I wouldn't advise to go for product shipping, go there in person, test the product and ensure all is in good condition before making payment.
If you're referring to second hand products, refurbished, or UK/US used products, yeah, you can get that also on Jiji or Techsourcesng. For the latter, check their IG page and see if they are offering what you're looking for. I have bought 3 laptops personally from them. Once again, always go in person to check things out before making payment.