I've been a member of ebay since about 2002 and have bought several sets of alloy wheels during that time. Many of the wheels, however, have left me less than happy. I'd say alloy wheels have been my most troublesome item on ebay! Hence, I thought I'd write a guide to share my experience.
Part 1- Sizing: When buying alloys you need to get your sizing right.This choice will often be about styling, and that's really your choice, but it will also affect your car's resale value and handling/steering responses. The initial sizing decision is wheel diameter (eg 17") and wheel width (eg 8"). Most standard alloys are around 7" in width.You can also go for a staggered set-up where the rears are 1" wider, for example.There are no major problems here unless you are going for sizes very different from standard. If you do change the diameter from standard you will need to check the tyre size has a similar outer diameter to the standard sizes (for example, 225/45/17 has the same tyre diameter as 225/40/18). The alloywheels.com website has an excellent tyre size calculator to help you sort this out. The tyre size calculator will also tell you what size wheels a particular tyre size will fit on. For example, a 225/45/17 tyre will fit on wheels from 7" to 9". Also, it's worth noting that wider tyres and lower profile tyres will alter the ride quality and steering responses (sometimes for the better). The other main sizing issue is Offset. This is probably the most imporant thing to get right. This is a measurement in mm which tells you how far the wheels stick out towards the kerb. The offset of the Mercedes W204 C Class, for exmple, is 45mm (ET45); a change to wheels with a 35mm offset would make the wheels stick out 10mm more towards the kerb (which may look better, but will alter the steering). Also, wheels with a higher number offset than standard may not fit in the arches - the inside wall of the tyre may rub on the brakes or suspension. The other sizing options are stud number, distance between studs and the bore diameter - this shouldn't be an issue if you are buying wheels for your make of car as most manufacturers keep this consistant. For example, Mercedes use 5 studs, 112mm apart with a bore of 66mm. In summary, get the offset more or less right unless you know they will fit; avoid increasing the offset; change the tyre sizes to match standard external diameter (more or less).
Par 2 - New or used: I often buy used alloys as they often get tyres thrown in with them. But I've had a lot of problems! Old alloys may be a little bent and won't balance up perfectly (I've had this problem).The condition might not be in line with the seller's description (I've had this problem).Wheels may have had a poor quality refurb in the past which can be hard to spot in pictures (I've had this problem). Wheels with poor paint finishes won't look good for very long. There can even be damage or repairs that the seller doesn't know about (I've had this problem) or they forget to tell you. These days I try to look at second hand wheels in the flesh or try to be careful anyway. Don't forget to look at the inside edges of the wheels too as these can have cracks and dents (Yes, had this too!).
Part 3 - Manufacturer. Wheels can be genuine (Original Equipment or OE), aftermarket (eg BBS) or copies (eg AMG reps). The manufacturing of all these is similar; the main difference is in the quality of the paint finish. The manufacturing quality of OE wheels or branded wheels should be fine, but the copies market is a mixed bag. There are well made copies out there, but there are also poorly made ones which may crack if bumped. If you want genuine wheels, I'd look for second hand due to the high cost - see alloywheelsdirect.net for prices. Decent sets do appear on ebay regularly and a good set will look great on your car. The same goes for high quality brands like BBS and OZ, but it's often difficult to find the style you want. Also, it's not always clear whether a certain style will look good on your car, whereas its easy to check what OE wheels look like. These days on ebay you can get decent looking copies for £300-400, which is reasonable I'd say, but the finish is not going to last as well as OE or top brands like BBS or OZ. Also remember the car market doesn't like non-standard wheels, so keep the originals and put them back on when you're done. You may get away with it if you're using copies as they should look like OE wheels, but it could spoil a sale. It is possible to get wheels with tyres delivered at reasonable cost with firms like Paisley Freight. The great thing about ebay is you can buy some wheels, use them and then sell them on again. At present I'm looking for second hand OE 18" AMG alloys, but am tempted by some nice new 18" AMG copies. Can't decide! Hope this guide help you.