Is it safe to buy a one time UPC / EAN number from one of those UPC / EAN Resellers?We do not recommended purchasing so called "one time UPC" codes or to use "buy a single barcode" services that are frequently advertised on the Internet (just google for "Code UPC"). Those codes do not carry your manufacturer prefix but the prefix of the respective service. Basically those vendors let you use you one of the numbers from their own number pool, exploiting a legal loophole with regard to UPC numbers.
There are two major problems with this approach:
1. If the service goes out of business or fails to pay their annual fee your numbers may become unusable.
2. The manufacturer prefix is also part of the SSCC-18 barcode on standardized shipping labels as used by virtually all major stores in the world. The prefix in the SSCC-18 code is used by the recipient to identify the sender (you). That's not possible in this case because you are using the reseller's prefix. There might even be number collisions if other companies that got their UPC codes from the same service happen to create an identical SSCC-18 code - a big no-no!
Also, registering with the GS1, obtaining a number pool, and purchasing a dedicated barcode software can actually be cheaper. The upfront cost may be higher but don't forget that you need an individual UPC number for all variations of a product, e.g. if you sell paperclips in lots of 100, 500 and 1,000 pcs. you'll need a UPC for each of them.