What kind of envelopes are you talking about and what quantities, tens of thousands or an assortment of smaller quantities? This site could be a good place to sell them.
I would suggest listening what you want to get rid of and at what price. I can’t speak for all distributors, but I know Lindenmeyr would sock you with a considerable restocking fee if you bought the envelopes from them. This was well over a year ago, but a sales rep couldn’t even approve a return for any paper without management approval. Too many larger printers hoarded too much and expected the paper distributor to take back what they over bought.
If you’re talking about large quantities of common sizes you may want to talk to your local paper representative that you have a good relationship with and see if they’re willing to broker a sale between you and another nearby printer. Transportation costs will be your enemy unless you can find someone in your local market. If these are smaller quantities of specialty envelopes, try reaching out to independently owned copy/print shops. Personally, I would have a hard time paying up front for envelopes that have been sitting around for years, I would want to inspect prior to delivery and payment.