When you're looking at buying mobile phones on eBay there are several other things to look out for on top of the reputation of the seller. The first is of course the quality of the item itself. Often you will be able to buy used phones or reconditioned phones which have been repaired, but if you are, the item should be listed as such. Read all of the small print before you make a bid and make sure that the item advertised matches up with the phone that you're looking for. Some sellers will deal exclusively in mobile phones and may offer guarantees and warranties on new or reconditioned products, which is always sensible to opt for if you're purchasing a mobile phone without actually having seen it in action before.
Second, check the type of listing that the item has been placed under. Some items are straight up auction room style listings, with or without a reserve on the price and you'll have to enter the maximum amount that you're willing to pay for it in order to be in the running. Others can be bought immediately for a fixed price, eliminating the need to bid. This may well be the case with less sought after mobile phones, though the latest models may often be put up for auction to initiate a bidding war and drive the price high, so don't get drawn in for more than you can afford. When you're bidding for a mobile phone and you enter the maximum price you wish to pay that won't automatically be your bid, but eBay will automatically raise the bid to match the other bids offered by competing customers and will never exceed your maximum.
Also be sure to keep an eye on the length of the listing, as some items have a matter of hours left before bidding closes and a winner is chosen and you could put in a late bid to clinch the mobile phone, accessory, or dongles of your choice.