eBay item specifics are the labelled details of a listing, such as brand, size, colour and model. Buyers use them as search filters and eBay names them as part of a complete listing. Fill in every required and recommended specific with accurate values from eBay's own lists, and you appear in more filtered searches.
Cassini is the name sellers use for eBay's search engine, which sorts results by Best Match. eBay says Best Match weighs how well a listing matches the search, item popularity, price, listing quality and completeness, listing terms such as returns and handling time, and the seller's track record. eBay does not publish the weights.
eBay Best Offer lets buyers propose a lower price on a fixed price listing, which you can accept, decline or counter. Switch it on for items with healthy margins or slow sales, set an auto-decline price at or above your minimum profit, and an auto-accept price you are happy with, so you never sell at a loss by accident.
The best categories to sell on eBay are the ones where buyer demand is steady, your profit survives category fees, returns are manageable, brand rights are not a minefield and you can deliver on time. Home and garden, pet supplies, DIY, car parts and hobby items often suit resellers, but score each category on your own data before committing.
Good AliExpress to eBay UK product research starts on eBay, not AliExpress: confirm that UK buyers already buy the product and at what price, rule out branded and heavily regulated items, work out the landed cost including shipping and import VAT, and test with samples before buying stock to hold in the UK.