He easily could have bought parts for a better price seeing as his has the hook-up through his employer.

Without further detail none of us can know the results. Calling someone a thief is a bit of a move…

I read it multiple times and fail to understand this interpretation at all. Even in context, I don’t see even a drop of “reasonable suspicion”.

Is it a possibility that the goods were stolen? I suppose, but that’s the case with literally anything you purchase online. I wouldn’t have even thought twice about it. I bought stuff, you didn’t send me stuff, so now I’m upset and want you to send me my stuff.

A more reasonable interpretation is that he was attempting to steal goods or time from the customer by dragging his feet on shipping; 'sorry, item is backordered' often results in the sale being lost, and salespeople are known for sometimes making promises to close deals, without regard to whether they can actually deliver.

Explain the knowingly part. I never suspected he did not own them or that they were stolen. Just knew that he took my money and didn't ship a complete order.

They could also have been counterfeit or substandard.

Not saying you're wrong (because I think you're right), but there are other scenarios here, so from a theft perspective, the OP would be not guilty.

sounds more like he was competing against his employer

It's not like it's uncommon for folks to leverage employee discounts as arbitrage opportunities for a side hustle. Maybe it violates their terms of employment since they're competing with their employer, but it's not stolen goods.

Why assume that person was stealing anything from the employer, rather than simply being a shitty reseller that only ships when they get a good discount from working in the industry.

A call like that can incentivize them to buy at full price and sell at a loss when their inventory is lacking.