Any return is damaging to all businesses in terms of cost as there's no way to avoid that. If the return is due to something that the seller has done wrong, such as misdescribing an item, then it could be said that they get their just desserts.
If the return is due to a "change of mind" reason then the seller doesn't lose as much as for a not as described return (unless they choose to offer free change of mind returns) as the buyer pays their own return postage. But the seller still has to refund the outward postage along with the item price, with no way to recoup that, so they end up down a certain amount of money (not to mention time) as well as losing the sale that they thought they'd made.
They will also have spent time researching, photographing, listing, packaging and posting the item only to have to start the selling process again once they receive the item back. What would be a violation of consumer rights would be for the seller to refuse to accept returns. All business sellers are legally required to acccept returns, even for a change of mind reason, so there is no option for them to state no returns. That would be illegal.
The only action a seller can take to try to protect themselves from potential repeats of time and money losing returns is to block anyone (if they wish) from buying in the future. If a seller feels that the chances of a buyer repeatedly returning items is high then they are free to decide not to sell to them again. Any seller is free to choose whether to sell to any buyer for any reason.
In your case, the item was an auction which you bid up to over £110. I don't know the reason for your return but, if it was a change of mind reason, then the seller is probably fed up at having to start the whole auction process again, having potentially lost the interest of buyers who bid on it last time who will probably be suspicious if they see it relisted. The fact that you didn't return the £13 item is little consolation. In this situation, a lot of sellers will block the buyer just to avoid any risk of a repeat of the hassle.
Even if it was a seller at fault return reason, the seller may disagree with whatever issue you had, so may not see it as a problem of their own making, and may still feel equally hard done by even if they were really at fault.
Ebay, in general, is very weighted in the favour of buyers rather than sellers. In some respects, it needs to be that way to maintain buyer confidence in using the platform, especially for expensive items. So buyer protections are extremely good. But sellers are free to block any buyer for any reason - or even for no reason at all if they want to. If the seller doesn't want to sell to you again then you'll just have to accept that. In terms of apportioning blame and taking sides, we don't know any of the details so that's impossible even if we wanted to.
